nanotechnology air force

DOD Researchers

Provide

A Look Inside

Nanotechnology

Special

Issue

QUARTERLYVolume 6, Number 1

A M P T I AC is a DOD Informa tion Analys is Center Adm inis te red by the De fen se Info rma tion Sys tems Age n c y, De fen se Te chnical In fo rmation Cent er and Opera ted by I IT Re s e a rch Inst i t u te

One of my colleagues from the DOE made the comment a fewmonths ago over dinner that if his proposals did not have theterm “Nano/Info/Bio-Tech” in the title, there was little chanceof them getting funded. We all laughed and commented onhow sometimes the buzzwords of the moment influence ourre s e a rch efforts more than the warf i g h t e r, defense needs,national interests, or for that matter, intellectual curiosity. Howmany times have you written a proposal and carefully crafted

the title to utilize a word or phrase that at the time was recog-nized as a buzzword, just to grab a sponsor’s attention?

I am afraid we are all guilty, and in some respects, nanotech-nology very effectively fits this category. You see it all over thenational media, from ABC to TLC, from USAToday to theWall Street Journal, and from Time Magazine to ScientificAmerican. And now this special issue of the AMPTIACNewsletter has landed on your desk, filled with information onnanotechnology. Thus, one could say that we too have fallenprey to the latest pop-culture trend of science.

Now that I have said what many of you may be thinking, letme explain why we have dedicated significant portions of thelast 12 months to generating the thick, full color document youhold in your hands, and why we hope that you will come awayfrom this with a better understanding of nanotechnology andwhat it means to the DOD in the 21st century.

People have worked with the nanoscale for centuries, butuntil recently lacked the analysis capability to understand itsimpact. The 20th century dramatically changed that, first withground-breaking tools for investigating the microscale, theneven more astounding tools that enabled investigation of indi-vidual atoms. Armies of engineers have driven advances in pro-cessing capabilities which have enabled first micro-, then nan-otechnology to move from science fiction to engineering fact.Much like the assembly line revolutionized conventional man-ufacturing in the early 1900s, batch fabrication and bulkmicromachining have again turned industry on its ear. Thepace of these changes is increasing, with nanoprocessing alreadyin existence and growing at an exponential rate. Some used tobelieve that a handful of computers would be enough to satiatethe world’s needs. Others believed that sequencing the humangenome would take decades, but it was finished in 15 years.

Today many people believe that molecular-level manufacturingis impractical, but there are materials and structures being pro-duced right now that we don’t even know how to test or ana-lyze yet.

With the help of Dr. James Murday (Superintendent of theChemistry Division at the Naval Research Lab, and Director ofthe White House initiated National Na n o t e c h n o l o g yCoordinating Office) we have recruited a stellar team of DODresearchers to explain how this field is making real contribu-tions now, and what its potential contributions will mean to thedefense of our Nation in the future. In clear, unambiguousterms, these authors have carefully laid out key aspects of thescience, physics, electronics, chemistry and engineering that areadvancing nanoscience at a blistering pace. From relatively sim-ple coatings with five times the toughness and wear resistanceof conventional materials, to the building blocks of optical andmolecular-based computers that are generations ahead of cur-rent silicon-based machines, you will see that nanotechnologyis more than the latest “flavor of the month.”

If you are a regular reader of the Newsletter, then you alreadyknow that AMPTIAC’s primary mission is to be a voice for thedefense materials and processing community. We therefore pro-vide you within these pages a snapshot of what the DOD isdoing to utilize nanotechnology and nanomaterials in practicalterms to improve the readiness of the warfighter. After readingthis issue, you will see that today’s nanoscience is laying thefoundation for the future of nanotechnology, transitioningmany science-level advances to technology applications in thenear and long term. Nanotechnology is more than just a buzz-word; it is a natural extension of the history of engineeringinnovation that has brought us to this point in time. When thevisions of our researchers and the promise of this field are real-ized, nanotechnology could very well change the foundationsof engineering, manufacturing and culture.

Wade BabcockTechnical Content Manager

Editorial: Nanotechnology is MoreThan the Latest EngineeringBuzzword

Cover Photo Credits: D.W. Brenner, J.D. Schall, and O.A.Shenderova, Department of Materials Science and Engineering,

North Carolina State University, Raleigh, NC 27695-7907

Special Issue: Nanotechnology

The Coming Revolution: Science and Technology of Nanoscale Structures … 5

Dr. James S. Murday, Executive Secretary, Nanoscale Science, Engineering and Technology Subcommittee, US National Science andTechnology Council and Superintendent of the Chemistry Division, Naval Research Laboratory

At the most basic level of common understanding, nanoscience involves the study of materials where some critical property is attrib-utable to an internal structure with at least one dimension less than 100 nanometers. This is truly the last frontier for materials sci-ence. As "nanotechnology" appears ever more often in the technical and popular media, defense researchers tackle the science andtechnology that will transform nanoscience into practical technology. Dr. Murday provides an overview of the efforts of thePresident’s National Nanotechnology Initiative, its accompanying work within DOD, and what they mean to the military, ouradversaries, and the future of this exciting field.

Emerging Technologies and the Army … 11

Col. Kip Nygren, Professor and Head, Civil and Mechanical Engineering, US Military AcademyTechnology is changing rapidly and often outpaces humanity’s ability to comprehend its advance. We, as humans, have alwaysrelied on our superior abilities to gather and process information and make proper decisions in a timely manner. This skill spansthe ages from stalking larger mammals in a hunt, to defeating our enemies on the field of battle. Now more than ever, success onthe battlefield is dependent on the rapid access to information and the ability to act on that data. Changing technology presentssome tremendous opportunities as well as pitfalls. Col. Nygren delves into the world of advanced technology and how it is shap-ing tomorrow’s warfighter.

Polymer Nanocomposites Open a New Dimension for Plastics and Composites … 17

Dr. Richard Vaia, Materials and Manufacturing Directorate, Air Force Research LaboratoryHumans place great importance on materials when talking about the past, from types of manufacturing to even more fundamen-tal conventions of naming specific epochs after the materials used (i.e. Stone Age, Bronze Age, Iron Age). Today’s frontiers of mate-rials technology are most definitely rooted in the combination of various materials to achieve specific goals with the greatest effi-ciency of properties. Dr. Vaia shows us how advanced plastics and composites designed for extreme service and environments areblazing a trail for tomorrow’s incredible advances.

Power from the Structure Within: Application of Nanoarchitectures to Batteries and Fuel Cells … 25

Dr. Richard Carlin, Director of the Mechanics and Energy Conversion Division, Office of Naval ResearchDr. Karen Swider-Lyons, Chemistry Division, Naval Research LaboratoryProbably one of the most established areas of nanotechnology is the use of nanomaterials in power generation and storage. Whilehighly dispersed nanoscale platinum particles have been used as electrocatalysts in fuel cells for years, the use of nanomaterials instorage and generation is far from fully exploited. Each year, researchers push the envelope with advances in control and modifica-tion of nanoscale properties in electrode structures. Drs. Carlin and Swider-Lyons explain some of the most recent advances in thestate of the art.

MaterialEASE: Materials Engineering with Nature’s Building Blocks … 31Richard Lane, Benjamin Craig and Wade Babcock, AMPTIAC Technical StaffMay we suggest that you read this article first? Messrs. Lane, Craig and Babcock provide a comprehensive primer which introducesthe science of the nanoscale. The text is written at a level any reader, from the experienced nanotechnologist to the layperson, willappreciate. Many basic aspects of the technologies described in the other articles in this issue are also explained in clear, conciseterms. This is definitely a good starting point for the non-“nano-savvy” reader.

The AMPTIAC Newsletter, Spring 2002, Volume 6, Number 1

A M P T I AC is a DO D Inform at ion Anal ysis C enter Adminis te red by the Defense In form at ion S ystems Age n c y, Defense Te chnical Inf ormat ion Center and Opera ted by I IT Re s e a rch Ins t i t u te

Nanoceramic Coatings Exhibit Much Higher Toughness and Wear Resistance than Conventional Coatings … 37

Dr. Lawrence Kabacoff, Materials Science and Technology Division, Office of Naval ResearchMost military and commercial applications re q u i re surface coatings which can resist wear and corrosion. Often taken for granted,these coatings can dramatically affect the standard service intervals for machinery, useful life of large components, and overall re a d i-ness of vital systems. While most ceramic coatings do wear ve ry slow l y, they usually fail from a lack of toughness and not a lack ofwear resistance. Dr. Kabacoff describes an innova t i ve processing method which utilizes traditional equipment to deposit films ofi n t e r m i xed nano- and microscale grains which stand up to we a r, but provide significant improvements in toughness and durability.

Nanoenergetics: An Emerging Technology Area of National Importance … 43

Dr. Andrzej Miziolek, Weapons and Materials Research Directorate, Army Research LaboratoryFrom the first experiments with gunpowder and fireworks to the latest ammonium nitrate and powdered aluminum high explo-sives, man has sought to unleash the force of chemical explosives in more powerful and controlled ways. Nanotechnology allowsresearchers to bridge the gap between pure chemical evaluation and microstructural analysis, and better understand the phenome-na which make energetics work. Dr. Miziolek presents a guided tour of some of the most groundbreaking work going on today inenergetics and how nanoscience is improving our understanding of one of our oldest weapons of war.

The Army Pushes the Boundaries of Sensor Performance Through Nanotechnology … 49

Drs. Paul Amirtharaj, John Little, Gary Wood, Alma Wickenden and Doran Smith, Sensors and Electron Devices Directorate, Army Research LaboratoryThe battlefield is a place where too much information is rarely a problem. Our soldiers need every bit of data that can be collect-ed and the field of available sensors and sensing systems is growing every day. An elite team of ARL researchers present some of thelatest thinking in sensor technology and describe how nanotechnology is changing the way sensors are designed, powered, deployedand utilized in the battlespace.

Fabricating the Next Generation of Electronics from Molecular Structures … 57

Dr. Christie Marrian, Microsystems Technology Office, Defense Advanced Research Projects AgencyComputers today are fabricated primarily from silicon, its oxides and nitrides, and thin films of metals, all deposited with pat-terning technologies that are quickly approaching a physical limit of resolution. What if the next paradigm of computing was basednot on the solid, electron conducting paths of silicon and metal compounds, but on molecules and the very atomic structures thatmake up our own brains? Dr. Marrian takes us to the limits of molecular-based computing with healthy doses of science-fact andpractical evaluation of components and systems that may very well be the next revolution in computing.

Engineering the Future of Nanophotonics … 63Dr. Bob Guenther, Physics Department, Duke UniversityDr. Henry Everitt, Associate Director of the Physics Division, Army Research OfficeOptical components are already established as the foundation for tomorrow’s telecommunications systems, and are quickly becom-ing de rigueur in next generation computing technology. In order for this to move from the world of science to technological appli-cation, new ways of controlling, transmitting and conveying photonic information will be developed. Drs. Guenther and Everittexamine the world of nanophotonics and show some of the systems that will transmit, generate, and indeed compute with light.

The AMPTIAC Newsletter is published quarterly by the Advanced Materials and Processes TechnologyInformation Analysis Center (AMPTIAC). AMPTIAC is a DOD sponsored Information Analysis Center,operated by IIT Research Institute and administratively managed by the Defense Information Systems Agency(DISA), Defense Technical Information Center (DTIC). The AMPTIAC Newsletter is distributed to morethan 25,000 materials professionals around the world.

Inquiries about AMPTIAC capabilities, products and services may be addressed to Dav id H. RoseDirector, A MPTIAC3 1 5 - 3 3 9 - 7 0 2 3E M A I L : a mp t i a c @ i i t ri . o rg

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Editor-in-ChiefChristian E. Grethlein, P.E.

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Introduction

The term “Nanoscience” is appearing in the technical literatureat an increasing frequency and each reference offers its own def-inition, some of which are better than others. At the most basiclevel of common understanding, nanoscience involves materi-als where some critical property is attributable to an internalstructure with at least one dimension limited to between 1 –100 nanometers. Below that size, the disciplines of chemistryand atomic/molecular physics have already provided detailedscientific understanding. Above that size scale, the last 50 yearsof condensed matter physics and materials science have provid-ed a detailed scientific understanding of microstructures. So, insome respects, one might consider the nanoscale to be the last“size” frontier for materials science.

The interest in nanostructures extends beyond their individ-ual properties. We have learned to exploit the natural self-assembly of atoms/molecules into crystals. The directed self-assembly of nanostructures (one, two and three dimensioned)into more complex, hierarchical systems is also an important

1980 1985 1990 1995 2000

Year

Figure 3. Nanoscience

Literature Citation Counts

Dr. James S. MurdaySuperintendent, Chemistry Division, Naval Research Laboratory

Executive Secretary, National Science and Technology Council’s Subcommittee on Nanoscale Science, Engineering and Technology

Figure 1. Paleontology

of Nanostructures

The AMPTIAC Newsletter, Volume 6, Number 1 5

Empiric Epoch Surface Epoch Nano Epoch

1960 1990

Nanolayer

Nanofilaments

Nanodots

Scanning Tunnelling Microscopy/Atomic Force Microscopy

Nanolithography (x10)

Self Assembly (x10)

Nanostructure (x10)

10000

9000

8000

7000

6000

5000

4000

3000

2000

1000

0

Figure 2. Superlattice

Devices Formed through

One-Dimensional Nano-

Technology Processing

High Electron Mobility

Transistor (HEMT)

High frequency receiver/

transmitter receiver

Quantum Cascade (QC)

Laser Gas Sensor

Vertical Cavity Surface

Emitting Laser (VCSEL)

Fiberoptic data commu-

nications, optical sen-

sors, encoder, range

finderHoneywell VCSEL

NRL InAs/AlSb HEMT

Lucent Technologies

goal. Without direct self-assembly, manufacturing costs wouldseverely limit the impact of nanotechnology.

While a scientific understanding of nanostructures is incom-plete, their use in technology dates back at least two thousandyears. The Lycurgis cup, a Roman artifact pictured in the lowerleft of Figure 1, utilized nanosized gold clusters to provide dif-ferent colors depending on front or back lighting. The Romanartisans knew how to achieve the effect, but were unaware of its

nanocluster basis. In the 20th century, nanos-tructures contributed to many significant tech-nologies. Examples of these include the additionof nanosized carbon particles to rubber forimproved mechanical properties (tires), the use ofnanosized particles for catalysis in the petro-chemical industries, and the nucleation of nano-sized silver clusters during photographic filmexposure. As depicted in Figure 1, one mightassign these examples to an empiric epoch in thecontinuing evolution of nanotechnology.

Empirically-based technology, withoutg reater scientific understanding, is usually diffi-cult to extend or control. The scientific founda-tion of nanostru c t u res re c e i ved a quantuma d vance when surface science enjoyed a re n a i s-sance, starting in the 1960s. Su rface science con-strained one material dimension into thenanometer size scale. Events catalyzing that re n-aissance we re the development of new surf a c e -s e n s i t i ve analytical tools, the ready availability ofultra-high vacuum (a by - p roduct of the space

race), and the maturity of solid-state physics (surfaces re p re-senting a controlled lattice defect – termination of re p e a t i n gunit cells). The principal economic driving force was the elec-t ronics industry, but surfaces we re also re c o g n i zed to play anessential role in many “re l i a b i l i t i e s” – adhesives, corrosion pro-tection, friction, we a r, fracture, etc. From 1960 to the pre s e n t ,s u rface science has pro g ressed from “clean, flat and cold” intothin films (two or more nanoscale interfaces) and film pro c e s s-

Table 2. Characteristic Lengths In Solid State Science Model

Field Property Scale Length

Electronic Electronic Wavelength 10.-- 100nm

Inelastic Mean Free Path 1.-- 100nm

Tunneling 1.-- 10nm

Magnetic Domain Wall 10.-- 100nm

Exchange Energy 0.1 -- 1nm

Spin-Flip Scattering Length 1.-- 100nm

Optic Quantum Well 1.-- 100nm

Evanescent Wave Decay Length 10.-- 100nm

Metallic Skin Depth 10.-- 100nm

Superconductivity Cooper Pair Coherence Length 0.1 -- 100nm

Meisner Penetration Depth 1.-- 100nm

Mechanics Dislocation Interaction 1.--1000nm

Grain Boundaries 1.--10nm

Nucleation/Growth Defect 0.1 --10nm

Surface Corrugation 1.-- 10nm

Catalysis Localized Bonding Orbitals 0.01 --0.1nm

Surface Topology 1.-- 10nm

Supramolocules Primary Structure 0.1 --1nm

Secondary Structure 1.--10nm

Tertiary Structure 10.--1000nm

Immunology Molecular Recognition 1.--10nm

Table 1. Carbon Nanotubes: Prediction and Subsequent Confirmation

Prediction Experimental Confirmation

• Armchair single wall nanotubes (SWNT) metallic • Individual single-wall carbon nanotubes as quantum wires

Mintmire, Dunlap, White, Phys. Rev. Let. 68, 631 (1992) Tans, et al. Nature 386, 474 (1997)

• All metallic SWNTs have a linear dispersion relation e(k) near the • Two-dimensional imaging of electronic wavefunctions in carbon nanotubes

Fermi level with a slope in k vs.e of –1.88 nm-1 eV-1 Lemay, et al. Nature 412, 617 (2001)

Mintmire, Dunlap,White, Phys. Rev. Let. 68, 631 (1992)

• Elastic modulus should be similar to high stiffness in-plane modulus of • Nanobeam Mechanics: Elasticity, Strength, and Toughness of Nanorods

graphite, Robertson, Brenner, Mintmire, Phys. Rev. B. 45, 12592 (1992) and Nanotubes, Wong, Sheehan, and Lieber, Science 277, 1971 (1997)

• Bandgaps of semiconducting SWNTs should obey an analytic • Atomic structure and electronic structure of single-walled carbon nanotubes

relationship inversely proportional to SWNT diameter Odom, Huang, Kim, and Lieber, Nature 391, 62 (1998)

White, Robertson, Mintmire, Phys. Rev. B. 47, 5485 (1993)

• Bandgaps of quasi-metallic SWNTs are inversely proportional to the • Energy gaps in metallic single-walled carbon nanotubes

square of the SWNT diameter, White, Robertson, Mintmire, Ouyang, Huang, Cheung, Lieber, Science 292, 702 (2001)

Clusters and Nanostructured Materials, (Nova, NY 1996) p. 231

• Stability of metallic SWNT conduction channels in presence of • Recent Confirmation: Fabry-Perot interference in a nanotube electron

disorder, leading to exceptional ballistic transport properties waveguide Liang et al., Nature 411, 665 (2001)

White, Todorov, Nature 393, 240 (1998)

• Near the Fermi level all semiconducting (metallic) SWNTs with similar • Spatially resolved scanning tunneling spectroscopy of single-walled carbon

diameters will have similar densities of s tates with the positions of the nanotubes, Venema et al., Phys. Rev. B. 62, 5238 (2000)

first few peaks obeying a simple analytic relation, White, Mintmire,

Nature 394, 29 (1998)

The AMPTIAC Newsletter, Volume 6, Number 16

ing. Su rface science has provided the scientific underpinningthat enabled a key aspect of “Mo o re’s Law” – the rapid minia-turization of thin film electronics technologies. Su p e r l a t t i c edevices (see Fi g u re 2) are a manifestation of the rapidly grow i n gt e c h n o l o g y, where the individual layers are nanometers in thick-ness and the interfaces are controlled with atomic pre c i s i o n .

So, if nanotechnology has been around so long, why is it onlyn ow the current rage? The 1990s nanoscience renaissance hasclose parallels to the 1960 surface science renaissance. Fi r s t ,beginning in 1980, the discove ry and development of prox i m a lp robes, such as scanning tunneling micro s c o p y / s p e c t ro s c o p y,atomic force micro s c o p y / s p e c t ro s c o p y, and near-field micro-s c o p y / s p e c t roscopy have all provided tools for measurement andmanipulation of individual nanosized stru c t u res. It took 10-15years for reliable commercial instruments to come onto the mar-ket which then led to a rapid increase in re f e reed nanosciencepublications beginning in the early 1990s. Fi g u re 3 highlightsthese increases for several subject areas. The pro p e rties of indi-vidual nanostru c t u res can now be observed, rather than re l y i n gon ensemble-averaged values. In turn, those pro p e rties can bedefined in terms of composition and/or stru c t u res, and withsuch an understanding comes the possibility for contro l .

Second, in addition to the new experimental measurementcapabilities, computer technology is now sufficiently advanced(speed and memory capacity) such that accurate predictions,even based on first principles (see AMPTIAC newsletter Vol. 5,#2, page 1), are enabled for the number of atoms incorporatedin a nanostructure (see Table 1). Analytic predictions, madeyears before experimental measurements could confirm their

validity, are now being proven true by direct experimentation.There is no doubt that modeling and simulation will play aleading role in the nanotechnology race.

Third, the disciplines of biology, chemistry, materials, andphysics have all reached a point where nanostructures are ofinterest – chemistry building up from simpler molecules,physics/materials working down from microstructures, andbiology sorting out from very complex systems into simplersubsystems. If one expected to simply extrapolate the propertiesof nanostructures from the size scales above or below, thent h e re would be little reason for the current interest innanoscience or nanotechnology.

There are three reasons why nanostructured materials behavevery differently than their adjacent scales. 1) Large surface-to-volume or interface-to-volume ratios, 2) size effects (wherecooperative phenomena like ferromagnetism are compromisedby the limited number of atoms/molecules), and 3) quantumeffects. Many of the models for materials properties at themicron and larger sizes have characteristic length scales ofnanometers (see Table 2). When the size of the structure is inthe nanometer range, those parameters are no longer adequateto model/predict the property. As such, one can expect “sur-prises,” resulting in new materials behavior that may be tech-nologically exploitable.

Finally, there are several economic engines driving the inter-est in nanotechnology, with information technology (electron-ics), biotechnology (pharmaceuticals and healthcare), andmaterials being the most likely beneficiaries (see Table 3).

With these substantial scientific and economic opportuni-ties, it’s not surprising to find strong global interest in fosteringnanoscience, with the intent of accelerating scientific discoveryinto innovative commercial products. Table 4 compares esti-mated governmental funding in nanosciences around theglobe. From estimates of FY02 budgets, it is anticipated thatover $1.5B will be invested in nanoscale science and technolo-gy (S&T) in 2002. Care should be taken when comparing

Table 3. Potential Global Economic Impact

of Nanotechnology in 15-20 Years [1]

Materials; Materials Processing $340B

Information Technology Devices $300B

Pharmaceuticals/Biotechnology $180B

Chemical Manufacturing, Catalysis $100B

Aerospace $70B

Tools, Automation, Life Cycle Costs $20B

Healthcare - Diagnostics, Prosthetics $30B

Sustainability - Agriculture, Water, Energy $45B

Total ~$1T

Table 4. “Nanotechnology” Research Program Investment ($M)

1997 2000 2001 2002

USA 115 270 420 600

Japan 120 245 500 900

Western Europe 125 200 250 300

Other Countries

(FSU, China, Australia, others) 70 110 200 400

Total 430 825 1,400 >2,200

Table 5. US National Nanotechnology Initiative Program ($M)

(Funding Breakdown)

Category Lead FY00 FY01 FY02

“Fundamental” Research 87 140 *

Grand Challenges 71 125 *

Nanostructured Materials by Design NSF

Nanoelectronics, Optoelectronics, Magnetics DOD

Advanced Healthcare/Therapeutics NIH

Environmental Improvements EPA

Energy Conversion/Storage DOE

Transportation DOT

Microcraft and Robotics NASA

Instrumentation & Metrology NIST new

Manufacturing Science NSF new

CBRE Protection/Detection DOD new

Centers/Networks 47 66 *

Infrastructure 50 77 *

Ethical/Social Implications 15 21 *

Totals (est.) 270 429 600

* Figures not available at press


funding levels between different countries. For instance, oneestimate of nanotechnology investment in China is around$25M/year; however, the manpower costs in China are signifi-cantly less than in the US, therefore, their investment is not assmall as it might otherwise seem.

US National Nanotechnology Initiative

The US response to the nanoscale scientific and economicopportunities is the National Nanotechnology Initiative (NNI)[2]. The funding categories for the NNI are provided in theNational Nanotechnology In i t i a t i ve supplement to thePresident’s FY2001 Budget [3]. The annual investment levels innanoscience are presented in Table 5. It would be fair to saythat all of the S&T funded under the NNI is fundamentalresearch. On the other hand, the S&T activity designated as“fundamental” in Table 5 corresponds to NSF investmentwhere the principal motivation is the generic knowledge baseand education.

In the ‘Grand Challenges’ section of Table 5, the fundamen-tal research investments listed are selected with technologicalramifications in mind. The lead agency has the responsibilityfor surveying the investment in the designated Gr a n dChallenge and for identifying investment opportunities/short-falls. There are three new grand challenges in FY02; the last,‘C h e m i c a l / Bi o l o g i c a l / R a d i o l o g i c a l / Ex p l o s i ve (CBRE) forDetection/Protection’, seeks to enable the considerable oppor-tunities for nanostructures to impact homeland defense. Aswith any rapidly changing technology, there is some anxietyover potential economic, environmental and social disruptionby nanotechnology. In response, NSF has sponsored a work-shop to address these issues [4].

It is expected that nanoscience/nanotechnology will enablemany important advances at the boundaries between tradition-al disciplines. Interdisciplinary research must be fostered toaccelerate that expectation. Cutting edge research will alsore q u i re expensive instrumentation for measurement andmanipulation of nanostructures. The Centers/Networks and

In f r a s t ru c t u re categories aremeant to address these twopoints. Table 6 shows newCenters initiated under theNNI. The DOE Centers will be user facilities available to allscientists/engineers in a waysimilar to the DOE synchro t ro nradiation facilities.

The NNI focuses on nano-science as the enabler of nano-t e c h n o l o g y. Experience hass h own that it takes 10-20 ye a r sfor a scientific discove ry to leadto a commercial product. How-ever, among the surface science-stimulated technologies, thereare a number of products nowentering the market. T h e s eproducts are mostly the nano-

dots and nanowires that are the building blocks for systems, butthere are also composite materials. A representative listing ofthose products is shown in Table 7.

DOD Contributions to the NNI

The DOD has been investing in fundamental nanoscienceresearch for over 20 years. One of the early programs datingback to the 1980s was Ultra-submicron Electronics Research(USER). In 1997, the DOD identified several S&T topics withthe potential for significant impact on military technology.Nanoscience was selected as one of several special research area(SRA) topics. A website [5] and coordinating committee wereestablished. Its current membership is comprised of Drs.Gernot Po m renke (Air Fo rce), John Pazik (Navy), BobGuenther (Army), and Christie Marrian (DARPA). Further,each Service has a coordinating group to guide its nanoscienceprogram (see Table 8). There are many potential areas wherenanoscience/nanotechnology may significantly impact DODmissions. A selected listing is provided in Table 9.

Each Service has its own laboratory nanoscience programs(see www.nanosra.nrl.navy.mil). The Army efforts in nanoelec-tronics, nanooptics, organic light emitting diodes (OLEDs)and displays, sensors, and NanoElectromechanical Systems(NEMS) are centered at ARL Adelphi; the work on organicnanomaterials is largely at ARL Aberdeen. The Air Force nano-materials program is largely centered at Wright Patterson AFBin Dayton. This work is focusing on nanocomposites, inorgan-ic nanoclusters, nanophase metals and ceramics, nanotribology,nanobiomimetics and nanoelectronics. The Navy program iscentered at the Naval Research Laboratory in Washington, DC.NRL has just created a Nanoscience Institute with the goal offostering interdisciplinary research that cuts across the NRLorganizational structures (http://nanoscience.nrl.navy.mil/). Anew Nanoscience Building at NRL has been specially designedto minimize those noise sources that would limit measurementand manipulation precision. The new building will open in2003 (see Table 10). NRL is interested in collaborative projects

Table 6. Nanotechnology Centers

Center Title Investigator Institution

National Science Foundation

Nanoscale Systems in Information Technologies Buhrman Cornell

Center for Nanoscience in Biological & Environmental Engineering Smalley Rice

Nanoscale S&E Center for Integ rated Nanopatterning & Detection Mirkin Northwestern

Center for Electronic Transport in Molecular Nanostructures Yardley Columbia

Nanoscale Systems and their Device Applications Westervelt Harvard

Center for Directed Assembly of Nanostructures Siegel RPI

DOE

Integrated NanoSystems Michalske Sandia/Los Alamos

Nanostructured Materials Lowndes Oak Ridge

Molecular Foundry Alivisatos Lawrence Berkeley

NASA

Bio/Nano/Information Technology Fusion TBD in 02

Bio-Nanotechnology Materials and Structures for Aerospace Vehicles TBD in 02

Nanoelectronics Computing and Electronics TBD in 02

DOD

Institute for Soldier Nanotechnologies TBD in 02


with other Services that can exploitthis new building and its special-i zed nanoscience measure m e n t /manipulation equipment.Since some of the DOD nano-

science programs are nearly 20years old, one might expect transi-tion successes. One example fro meach service is illustrated here .

Under Army funding Dr. ChadMirkin, Northwestern University,invented a way to utilize nanoclus-ters of gold for the sensitive, selec-tive detection of DNA. This tech-nology has been demonstrated towork for anthrax. It is now beingc o m m e rc i a l i zed by the start - u pfirm Nanosphere, and is presentlyunder clinical evaluation.

The Air Force funded TritonTechnologies Inc. under the SBIRprogram to insert nanostructuredclay particles into polymers. Onebenefit of this composite is reducedgas permeability. This new materi-al was marketed by Converse inathletic shoe heels with gre a t e relasticity (He gas bubbles trappedby the low permeability polymercomposite). The reduced perme-ability is also of interest to theArmy for packages containingfood, beverages and pharmaceuti-cals. The Navy is interested becausenanoclay particles increase the fireresistance of organic compositematerials.

Table 7. Examples of Commercially Available Nanotechnology Products

Nanopowders

Altair www.altairtechnologies.com/ TiO2

Argonide Nanomaterials www.argnonide.com aluminum

Bosch www.bosch.de ceramics

BuckyUSA www.flash.net/~buckyusa/ fullerenes

Inframat Corporation www.inframat.com Yttria Stabilized Zirconia (YSZ), FeS,

n-WC/Co, Co/SiO2, Ni-Fe/SiO2

Nanomaterials Research Corp www.nrcorp.com oxides

Nanophase Technologies www.nanophase.com oxides

Nanopowder Enterprises www.nanopowderenterprises.com/ oxides, metals

NanoPowder Industries www.nanopowders.com/ precious/base metals

Nanox www.nanox-online.com/ ZnO

NexTech Materials www.nextechmaterials.com/ oxides

Plasmachem www.plasmachem.de diamond, alumina, SiC, Cu

US Nanocorp, Inc www.usnanocorp.com n-MnO2, n-Ni(OH)2, n-FeS2

Powdermet www.powdermetinc.com/ metals, ceramics

Nanomat Ltd (Ireland) www.nanomatgroup.com/ metal oxides

Nanowires/Nanotubes

Carbon Solutions (Sweden) www.carbonsolutions.se/ carbon nanotubes (CNT)

Carbolex www.carbolex.com single wall nanotube (SWNT)

Carbon Nanotechnology www.cnanotech.com/ SWNT

Hyperion Catalysis www.fibrils.com/ multiwalled nanotubes (MWNT)

Materials & Electrochem. Res. www.mercorp.com/ SWNT, MWNT

NanoLab www.nano-lab.com/ CNT and arrays

Sun Nanotech (China) www.sunnano.com/ MWNT

Piezomax www.piezomax.com/ Carbon Nanotube SPM Tips

eSpin www.nanospin.com/ polymer nanofibers

Nanocomposites

Inframat www.inframat.com/ magnetic nanocomposite

Nanocor www.nanocor.com/ clay in plastics

RTP www.rtpcompany.com/ CNT compounds

Southern Clay Products www.nanoclay.com/ clay in plastics

Triton Systems Inc www.tritonsys.com/ nanoparticles in plastics

Hybrid Plastics www.hybridplastics.com/ organic/inorganic hybrids

Table 8. DOD Points of Contact on Nanotechnology

Navy Nanostructures Working Group

Dr. Larry Cooper Electronics, ONR ([email protected])

Dr. Eric Snow Electronics, NRL ([email protected])

Dr. Khershed Cooper Materials, NRL ([email protected])

Dr. John Pazik Materials, ONR ([email protected])

Dr. Richard Colton Biology, NRL ([email protected])

Dr. Keith Ward Biology, ONR ([email protected])

Dr. Gary Prinz Nanoscience Institute, NRL ([email protected])

Dr. James Murday Chair ([email protected])

Air Force Nanostructure Working Group

Dr. Gernot Pomrenke AFOSR ([email protected])

Dr. Richard Vaia AFRL/ML (Materials/Manufacturing) ([email protected])

Dr. David Hardy AFRL/VS (Space Vehicles ([email protected])

Mr. Dan Burns AFRL/IF (Information) ([email protected])

Mr. Ross Detmer AFRL/SN (Sensors) ([email protected])

Dr. Jim Gord AFRL/PR (Propulsion) ([email protected])

Mr. Bill Baron AFRL/VA (Air Vehicles) ([email protected])

Mr. Rik Warren AFRL/HE (Human Effectiveness) ([email protected])

Dr. John Corley AFRL/MN (Munitions) ([email protected])

Dr. Don Shiffler AFRL/DE (Directed Energy) ([email protected])

Mr. Joe Orlowski AFRL/XP (Program Management) ([email protected])

Army Nanoscience and Nanotechnology Center

Dr. Andrew Crowson Coordinator, ARO ([email protected])

Dr. Paul Amirtharaj Sensors and Electron Devices, ARL ([email protected])

Dr. Gary Hagnauer Weapons and Materials, ARL ([email protected])

Dr. Raju Namburu Computation and Information, ARL ([email protected])


Table 9. Nanoscale Opportunities with Major

DOD Impact

Nanoelectronics/Optoelectronics/Magnetics

Network Centric Warfare

Information Warfare

Unmanned Combat Vehicles

Automation/Robotics for Reduced Manning

Effective Training Through Virtual Reality

Digital Signal Processing and Low Probability of Intercept

Nanomaterials “by Design”

High Performance, Affordable Materials

Multifunction Adaptive (Smart) Materials

Nanoengineered Functional Materials (Metamaterials)

Reduced Maintenance (halt nanoscale failure initiation)

BioNanotechnology – Warfighter Protection

Chemical/Biological Agent detection/destruction

Human Performance/Health Monitor/Prophylaxis

10

Under Navy funding, Inframat has developed a thermallysprayed coating of alumina/titania nanopowders. The proper-ties of this coating are far superior to the micropowder equiva-lent (see Table 11). This product was one of the R&DMagazine selections as an R&D 100 product of the year for2000 and it currently under field evaluation on Naval ships.

Conclusion

DOD programs in nanoscience are major contributors to theNNI. The other articles in this issue present some of those pro-grams and provide an assessment of the present state-of-the-artin nanoscience/nanotechnology, with particular emphasis onDOD needs. T h e re is clearly growing global interest innanoscience and nanotechnology, driven by the science oppor-tunities and the expectation of large economic benefits. Sincethe US will not dominate the advances in nanoscience and nan-otechnology, it is incumbent on the DOD to invest in thosenanoscience/nanotechnology areas important to its interests, toexpeditiously convert nanoscience discovery into innovativeDefense technologies (thereby maintaining technology superi-ority over opposition forces), and to pay close attention todevelopments outside the US.

Cited References

[1] Dr. Mihail Roco, NSF; also NanoBusiness Alliance[2] http://nano.gov; IWGN Workshop Report with input from aca-demic, private sector and government communities (also KluwerAcademic Publisher, 1999, ISBN 0-7923-6220-9; Tel:(781) 871-6600; email: [email protected])[3] http://nano.gov/nsetrpts.htm[4] “Societal Implications of Nanoscience and Nanotechnology” NSFReport, March 2001 (also published by Kluwer Academic Publishing,2001)[5] http://www.nanosra.nrl.navy.mil

General References to the data in Table 11

1. Y. Wang, S. Jiang, M. Wang, S. Wang, T.D. Xiao, P.R. Strutt,“Abrasive Wear Characteristics of Plasma Sprayed NanostructuredAlumina/Titania Coatings,” Wear 237 (2000) 176-185.2. R.W. Rigney, Presentation at ONR DUST Nanostructured CoatingProgram (ONR contract No. N00014-98-3-0005), Cape Cod, May17-19, 2000.3. M. Gell, “Ad vanced Coating Technology De velopment forEnhanced Durability and Reduced Cost in Naval Application,” ONRContract No. N00014-98-C-0010, June 1997-Dec 2001.4. T.D. Ciao, Y. Wang, S. Jiang, S. Wang, D.M. Wang, & P.R. Strutt,“Thermal Spray of Nanostructured Alumina/Titania Coatings withImproved Mechanical Properties,” Procs. 2nd Intl. Conf. SurfaceEng., Wuhan, China, Oct 17-22, 1999, pp12-146.

Dr. James S. Murday received a B.S. in Physics from Case Western Reserve in 1964, and a Ph.D. in Solid State Physics from

Cornell in 1970. He joined the Naval Research Laboratory (NRL) in 1970, led the Surface Chemistry effort from 1975-1987, and

has been Superintendent of its Chemistry Division since 1988. From May to August 1997 he served as Acting Director of Research

for the Department of Defense, Research and Engineering. He is a member of the American Physical Society, the American

Chemical Society and the Materials Research Society; and a fellow of the American Vacuum Society (AVS), and the UK Institute

of Physics. For the AVS, he has served as trustee for 1981-1984, director for 1986-1988, representative to the American Institute

of Physics Governing Board 1986-1992, president for 1991-93, and representative to the Federation of Materials Societies 1998-

present. His research interest in nanoscience began in 1983 as an Office of Naval Research program of ficer and continues

through the NRL Nanoscience Institute. He has organized numerous International STM/NANO conferences and their proceed-

ings. Under his direction, both the AVS and the International Union for Vacuum Science, Technology and Applications created a

Nanometer Science/Technology Division. He is Executive Secretary to the US National Science and Technology Council’s

S u b c o m m i t tee on Nanoscale Science Engineering and Te chnology (NSET) and Dire c to r, National Na n ote ch n o l o g y

Coordination Office.

Table 11. Properties of Inframat Nanoalumina/Titania Coating

(www.inframat.com)

Properties Conventional Nanostructured Improvement

Alumina/Titania Alumina/Titania

Toughness Poor Excellent Dramatic

Hardness (VHN) 1,000 VHN 1,000 –

Wear resistance

(N*m/mm3)7.5 x 103 40 x 103

~5X

Corrosion

ResistanceGood Exceptional Significant

Grindability Poor Excellent Dramatic

Fatigue Life Failure @ <1 No failure up to

million cycles 10 million cycles >10X

Flex Tolerance Will result in Can be bent to

coating spallation over 180 without Dramatic

any spallation

Bond Strength (psi) 1,900 ~8,000 >4X

Table 10. NRL Nanoscience Building Requirements

Class 100 Cleanroom – 5,000 sq ft, ballroom style

Chemical (hoods, wet processing stations,…)

Dry Processing (dual focused ion beam, chemical vapor deposition,…)

Photolithography (pattern aligner,…)

Nanolithography (e-beam writer, dip-pen lithography,…)

Quiet Laboratories – 8 @ 400 sq ft

EMI <0.3 mG pp 60 Hz

Vibration <3µm/sec rms 10-100 Hz

Acoustic <35dB @ 1kHz

Temperature ±0.5 °C, <0.5 °C/hr

Humidity 45±5%

Ultraquiet Laboratories – 4 @ 400 sq ft

EMI <0.3 mG pp 60 Hz

Vibration <3µm/sec rms 10-100 Hz

Acoustic <25dB @ 1kHz

Temperature ±0.1 °C, <0.1 °C/hr

Humidity 45±5%


The last decade of the 20th Century provided a glimpse of thespeed and magnitude of the revolutionary changes that willprofoundly transform society and the Army in the early part ofthe 21st Century. The emerging technologies that will be themajor contributors to this transformation are biotechnology,nanotechnology and robotics/artificial intelligence (AI). Thisarticle considers how these technological innovations couldsubstantially change the nature of warfare and more impor-tantly, highlight the Army’s need to overhaul its processes andorganization to cope with the acceleration of technologicalchange.

Emerging Technologies

Revolutionary technological change is coming, and two thingswill be different about this change than changes in the past –its magnitude and speed. The magnitude of change is going tobe significantly greater than we can presently envision. There isbroad agreement among experts about the general direction oftechnology’s evolution, and that the three emerging technolo-gies of biotechnology, nanotechnology, and robotics/Al aredriving the change. The information technology innovations ofthe 1990’s will look tame when compared to the technologicalrevolution that will occur during the first decade of the 21stCentury, wrought by the synergism of these three technologies.

BIO-ENGINEERING The sequencing of the human genomeand the genomes of other species have provided access to theblueprints for the construction of biological entities and we areat the early stages of learning how to make desired modifica-tions to those entities. This knowledge paves the way for indi-vidualized drug therapies, noninvasive surgery, tissue and organengineering, neural and sensory implants, biological comput-ing and countless other currently inconceivable possibilities. Itis clear that the quality and length of human life will be signif-icantly increased, and that the promise of improvementsbeyond current levels in all areas of human performance iswithin reach [1].

NANOENGINEERING In 1986, K. Eric Drexler published abook called the “Engines of Creation,” [2] which revealed his

vision of a practically utopian future that will be realized bymeans of revolutionary machines and structures built on themolecular scale. The size of these machines is measured innanometers (billionths of a meter) and they would theoretical-ly be able to manufacture any physical item, assembling it mol-ecule by molecule from the inside out. The objects producedwould be physically perfect because each and every moleculewould be placed in exactly the right position. The molecularbuilding materials would be obtained through the actions ofdisassemblers, taking apart source materials molecule by mole-cule for the assemblers to use in construction. No materialswould be wasted and, therefore, pollution would be kept to anabsolute minimum.

The construction of a complex device would require theassemblers to first replicate themselves to achieve a level of crit-ical mass permitting complete fabrication of any desired partwithin a reasonable period of time [3]. Figure 1 is a visualiza-tion of a nanoscale robot arm intended to move atoms in theassembly process. The dream of molecular nanotechnology is todo for the atom what information technology has done for thebit [4]. This is the long-term, optimistic vision for nanotech-nology, but even today methods of building nanoscale struc-tures and machines are achieving some interesting and usefulresults by building from the top down. A crystal is an exampleof the natural growth of self-organizing nanoscale atomic struc-tures, and similar properties are being exploited by researchersto produce a variety of nanostructures including electrical,

Col. Kip NygrenProfessor and Head, Department of Civil & Mechanical Engineering

US Military Academy

Most of the articles in this special issue of the Newsletter address definitions, functions, and applications of nanotechnology and nano-engineered materials. Colonel Nygren takes a broader brushstroke, providing us with a greater perspective on the rapidly changing worldof high technolog y, and what role nanotechnology will play in the decades to come. An insightful and provocative look into the first halfof the 21st Century! – Editor

Figure 1. Nanoscale Robot Arm

(Drexler, NASA Ames)


mechanical, electromechanical and quantum devices [5].Potential capabilities include the proliferation of sensors andactuators which could lead to “. . . clothes that respond to theweather, interface with information systems, monitor vitalsigns, deliver medicines, and automatically protect wounds; airfoils that respond to airflow; buildings that adjust to theweather;” and “bridges and roads that sense and repair cracks; . . .” [6].

Microelectromechanical systems or MEMS, measured inmillionths of a meter, are currently used in a variety of applica-tions, including the actuation of automobile airbags, fiber opticdata switching, and the direct integration of analog and digitalcircuits on silicon chips used in cellular telephones. As pub-lished recently in Physics Today, “. . . the field of micromechan-ics will change the paradigm of what machines are, how andwhere we use them, what they cost, and how we design them.It may not be an exaggeration to say that we are on the verge ofa new industrial revolution driven by a new and completely dif-ferent class of machines” [7]. While larger than nanoscale,MEMS devices are still quite small. A MEMS-scale gear is com-pared to red blood cells and a grain of pollen in Figure 2.

ROBOTICS/ARTIFICIAL INTELLIGENCE The steady shrinkage ofelectronic components will persist into an era of nanoelectron-ics and permit the processing power of computers to increasewell beyond that of the human brain. Computers already indi-rectly augment the processing capability of humans today, suchas when a word processing program helps to correct a writer’sspelling errors. For many years, technology has providedreplacements for bones and joints, augmentation for sight andhearing, and the support and replacement of damaged organs.

Augmentation of human memory and mentalprocessing through direct connection to comput-ers will simply be a natural progression of theseexisting trends [8]. Extreme miniaturization willenable people and machines to interact in waysthat are unimaginable except in science fictionnovels.

SYNERGISTIC COMBINATIONS As these technological advancescombine synergistically, they have the potential to positivelyand profoundly transform the world. Nanoscale intelligentrobots could augment and repair living cells leading to theextension of life and enhanced individual human performance.Within 25 years, computers with a processing capability mil-lions of times that of today’s computers will be capable of learn-ing on their own [9]. The insights and knowledge they maylearn from reading the entire expanse of human knowledge arebound to have momentous influences on our connections toand control of our environment.

Currently, bacteria are being designed by deliberate modifi-cation of their DNA to produce protein molecules for com-puter memory. Laboratories at the University of Connecticutand Syracuse University have designed and produced threedimensional, gigabit scale memory modules for the Air Forceconsisting of protein molecules suspended within a solid poly-mer matrix that can store electrons produced by photons of aparticular wavelength and release them on demand [10]. A similar three dimensional memory medium has also been created using molecular electronics [11]. These memories canstore information in a three dimensional array that is unaffect-ed by electromagnetic impulses and extreme accelerations.

Exponential Change

Why is this great change going to occur at such a rapid rate?Studies of technological change over the past century lead tothe conclusion that technology is evolving exponentially. Thisis not surprising since all evolutionary change is exponential,but what is surprising is the greater than expected rate of expo-nential technological change. Figures 3 and 4 show that manu-facturing productivity and per capita GDP have increased inapproximately an exponential manner over the past severalyears. A study of overall technological change by RaymondKurzweil indicates that technology is advancing at approxi-mately a seven percent rate of increase per year, or to put itanother way, the effectiveness of technology doubled every tenyears in the twentieth century [12].

In the first half of the last century, technology evolved overfive doubling periods to become 32 times more advanced (ormore complex, or more important in the life of humans) in1950 than it was in 1900. Now, continue the doubling ofprogress every ten years during the later half of the century andby the year 2000, technology had become about 1000 times

Figure 2. Micro Gear

with Pollen and Red

Blood Cells (Sandia

National Labs)


50µm

“We will experience an equivalent level of technological change in the first decade of the 21st Century to what we experienced in all of the 20th Century.”

more advanced, or important in ourlives, than in 1900. Astonishingly, halfof this change occurred in the lastdecade of the century and it was cer-tainly the 1990’s that brought the ideaof rapid technological change to theattention of most observers. It is thenature of exponential change to materi-alize faster than we expect it to, becausehumans tend to be linear thinkers.

IMPLICATIONS FOR THE FUTURE By2010, technology will be 2000 timesm o re advanced than in 1900.Therefore, we will experience an equiv-alent level of technological change inthe first decade of the 21st Century towhat we experienced in all of the 20thCentury! Extrapolating to the end ofthe 21st Century, one finds technologywill have progressed to become a mil-lion times more advanced than thetechnology that existed in 1900. Inother words, we will experience a thou-sand times the technological progressachieved in the 20th Century, duringthe 21st Century.

But what about such technologies ascars, airplanes, bicycles, the can opener,the mouse trap and many others thathave not appeared to advance much atall over the last few years, much lessexponentially? The history of techno-logical advance is the story of countlessdifferent technologies erupting expo-nentially onto the scene and then level-ing off in their growth or improvementin what is classically known as an Scurve of growth. It is the summation ofall technologies that is considered hereand that summation presents a global image of acceleratingchange. This aggregate of technological change is probably bestcaptured by such indices as manufacturing productivity and percapita GDP. These indicators are effectively exponential innature and therefore, provide strong support to the premisethat exponential change is a compelling model for understand-ing the accelerating advance of technology.

I mplications for the Army

Although the effects of emerging technologies on the futureArmy cannot be known, we will speculate about some real pos-sibilities for the future.

THE DECISION CYCLE Consider the potential to augment theespecially stressful and demanding decision-making processthat is at the heart of winning battles and wars. An effectivemodel of this process was developed by Colonel John Boyd ofthe Air Force from his observations of air to air combat inKorea. It is known as the Observation – Orientation – Decision– Act or O-O-D-A cycle. Observe - what is out there? Orient -what does that mean to me and what are my options? Decide -select the best course of action. Act - execute the course ofaction and begin the process over again [13]. Consider some ofthe ways this cycle can be enhanced by the emerging technolo-gies under discussion, and specifically with the AI toolCommander’s Associate.

Figure 4. US Per Capita GDP Change (US Dept of Commerce)

Figure 3. Manufacturing Productivity (US Dept of Commerce)


Per Capita US GDP Cur rent Year $

40,000

35,000

30,000

25,000

20,000

15,000

10,000

5,000

0

5.2% Growth Rate or14 year Doubling Period

Year

450

400

350

300

250

200

150

100

50

0

2.8% Growth Rate or 25 year Doubling Period

Year

Manufacturing Productivity Index per Year

OBSERVE – Sense. The Army is already using autonomousrobotic vehicles as observers in Afghanistan and these will onlybecome more reliable and effective as they are employed in theair, on the ground, and even under the ground. The miniatur-ization of machines, structures and electronics will make avail-able large masses of sensors of varied types that can be deployedover the entire battle area to continuously monitor all potentialenemy signatures. Automatic target recognition will relieve

some of the data processing burden and other capabilities mayinclude the obstruction and confusion of enemy attempts toobserve the telltale signatures of friendly forces.

ORIENT - Process the Information. The principal criticismheard about information technology in the Army is that com-manders and staffs cannot adequately process the vast quantityof information available in a timely manner, resulting in moreconfusion and less clarity. An AI tool known as theCommander’s Associate will soon be available to alleviate this sit-uation. This artificially intelligent assistant will assess andassimilate all the incoming data, draw tentative conclusions incollaboration with the commander and staff, and visually dis-play the important information on a map or in any otherdesired format. All this processing is accomplished nearlyinstantaneously and will be continuously updated.

Because the Commander’s Associate also knows all the friend-ly forces and dispositions, it can just as rapidly offer recom-mended courses of action. The commander will verbally inter-act with the AI associate to modify and consider a wide rangeof feasible alternatives to accomplish the mission. By means ofsensitivity analysis, the AI associate will be able to assess theimpact of assumptions, advise which operational phases havethe highest risk, and predict probable enemy reactions -- allaccomplished in close collaboration with the commander andhis staff. Once again, these outcomes will be continuouslyupdated as more data is received.

DECIDE – Select the best Course of Ac t i o n. As theCommander’s Associate becomes a trusted advisor and soundingboard for the commander’s ideas and judgments, it will be aninvaluable aide in laying out alternative courses of action,demonstrating the strengths and weaknesses of each, and final-ly presenting a recommended action with full justification.Once the decision is made, the AI associate can communicatethe decision and coordinating instructions to appropriate units

either electronically or physically by autonomous air or groundvehicles.

ACT – Make it happen. Most importantly, the AI assistantcan monitor all aspects of the action in real time and make rec-ommendations to modify any phases of the plan as it is execut-ed. The entire decision cycle could be completed in minutes ore ven seconds, as the commander desires, and can be continuously iterated by the AI to account for new data.

Boyd demonstrated that thecycle time is the critical fac-tor in determining successin combat. T h eCommander’s Associate willbe instrumental in assuringthe friendly decision cycletime is less than an oppo-nent’s cycle time, therefore,

driving the opponent to inevitably take action based on anobservation that no longer applies. This will rapidly lead anenemy commander to realize that he cannot influence the situ-ation in any coherent manner, and with any luck, in total frus-tration he will lose the resolve to continue the conflict.

Weapon systems needed to create the violence, or the threatof violence in combat, will change dramatically as a result ofemerging technologies. If we continue with the same basicweapon types as today, then molecular manufacturing shouldbe able to produce these weapons, equipment and structureswith minimum weight and maximum strength. Artificiallyintelligent and autonomous air, land and subterranean combatrobots in a wide range of sizes from nanometer to meter scaleswill be employed individually and in swarms in conjunctionwith soldiers to accomplish the mission.

SOLDIER SYSTEMS Soldiers will wear, carry and ride in a for-midable new ensemble of weapons, sensors, communicationsand protection systems. Each soldier will verbally communicatewith an artificially intelligent personal assistant computer.Similar to the Commander’s Associate, the personal assistant willhelp the soldier make sense of incoming data and offer recom-mendations for everything from movement routes over terrainto the call for support fire. The soldier assistant can monitor allsoldier biological functions, begin immediate and comprehen-sive first aid assistance when wounded, and notify medical per-sonnel when assistance is required. When appropriate, the AIassistant can enhance human biological performance systems toensure the soldier is functioning at optimum capacity in anysituation presented to him.

Other potential soldier systems include: [14]• Active chameleon style camouflage systems for a wide

range of the EM spectrum• Exoskeletons to augment human muscles• Ballistic protection 5 to 10 times more effective than


“The Commander’s Associate will be instrumental in assuring cycle timeis less than an opponent’s … driving the opponent to inevitably takeaction based on an observation that no longer applies.”

current systems with correspondingly less weight• Interactive textile/clothing that provides passive insula-

tion and cooling; chemical and biological agent detection, protection, and decontamination; self cleaning;and electromagnetic and radio frequency shielding

• Clothing and materials that provide energy harvesting,conversion and storage

• Individual waste disposal, water recovery and recycling

LOGISTICS AND MEDICAL SUPPORT Robotics and artificialintelligence will permit the continuous monitoring of the sup-ply and maintenance status of all equipment and units. As thelogistics system automatically detects that a unit requires resup-ply, robotic ground and/or aerial vehicles will be dispatched totransport the requisite classes of supply directly to the usingunit. With this degree of automation, some current levels of thelogistics system could be eliminated. With advanced biotech-nology, the soldier’s health and level of performance can becontinuously monitored and supported in much the same wayas equipment and materiel. Medics using teleoperated surgicalequipment can attend to serious wounds at the lowest echelon.The entire medical system will know almost instantly the statusof all soldiers and will be prepared to accommodate woundsand injuries as quickly and proficiently as possible.

TRAINING The foremost benefit for training provided byemerging technologies will probably be virtual reality simula-tions that permit the rehearsal of all combat operations at a veryhigh level of fidelity to include detailed depiction of terrain andenemy behavior. Individual and unit combat skills can be faith-fully honed in highly realistic surroundings against an enemydesigned to authentically respond under any environmentalconditions. Virtual reality training will be similar to the aircraftsimulators currently used to train airline and military pilots inall aspects of flight.

ACQUISITION The increasing speed of technological changewill compel a complete revision of the current military tech-nology acquisition processes. Technological innovations cannotbe developed in isolation from doctrine, organization, trainingand leadership. An integrated and continuous Army transfor-mation process must be established to optimize overall militaryeffectiveness for a variety of missions performed in all conceiv-able environments.

THE DARK SIDE Technology is and always will be a double-edged sword for society. As technology becomes more advancedon the positive edge of the blade, so will it possess more poten-tial for perils to society on the negative edge. In the wake ofrecent terrorist attacks, the design of new technologies requiresa revision of the public safety design constraint: Public safetynow has gained a higher priority in a new environment whereit can no longer be assumed that individuals will take no delib-erate actions that lead directly to their own demise. In some

cases, the public safety will demand a benign abort mode for alltechnologies with a potential for harm above a specified energyor lethality level. In other cases, access to the technology maybe severely restricted and ethical standards for design engineersstrictly adopted and rigidly enforced.

ETHICS AND ARMS CONTROL The importance of ethics andprofessional responsibility in engineering design cannot beoveremphasized when weapons of mass destruction can beinexpensively and straightforwardly created by anyone withmodest specialized knowledge and equipment. Arms controlstyle agreements offer one option for halting the spread of dan-gerous technology applications, but these agreements will notinclude non-state terrorist and radical militant gro u p s .However, arms control treaties would still be important tools torestrain the dark side of emerging technologies, and the Armycould provide the prime forces for verification of compliancewith international treaties and agreements. In the case of non-state sponsored militant groups, the Army could find itself amajor Homeland Defense Force team member, working close-ly with intelligence organizations to enforce United Nationssanctioned ethical standards and controls on research intounmistakably dangerous technologies; including infectiousbiotechnology products, malicious information technologyviruses, and other nefarious weapons.

We are only beginning to come to grips with the new mis-sion of Homeland Defense. It is a far-reaching mission thatinvolves extensive intelligence operations and sweeping require-ments for security and special operations forces. Emergingtechnologies can make substantial contributions to the protec-tion, decontamination and neutralization of biologically infec-tious living organisms and nanoscale mechanisms. Highly sen-sitive micro and nanotechnology laboratories, constructed onsilicon chips using the same methods as integrated circuits,could be inexpensively produced and even woven into clothingto provide detection and decontamination.

The Army Must Meet the Challenge

The accelerating advance and increasing complexity of technol-ogy will put enormous strains on the capability of the militaryto adapt to an uncertain future and still accomplish its core mis-sion of national security. The Army must embrace change andcontinuously transform itself not only technologically, but alsoin its organization, doctrine, training, and leadership. Ed u c a t i o nis the key to continuous transformation. Both individuals andorganizations must become increasingly better at learning andmodifying their behaviors in light of new knowledge andinsights. But this is another subject beyond the scope of this art i-cle. Howe ve r, the discussion must be joined now because theu n relenting advance of technology cannot be stopped. References

[1] Anton, P.S., R. Silberglitt, et al. (2001). The Global Technology



Re volution: Bi o / Na n o / Materials Trends and their Synergies withInformation Technology by 2015 . Santa Monica, CA, RAND.[2] Drexler, K.E. (1986). Engines of Creation: The Coming Era ofNanotechnology. New York, Doubleday.[3] Drexler, K.E. (2001). Machine-Phase Nanotechnology. ScientificAmerican. 285: 74-75.[4] Jacobstein, N. (2001). Technical Considerations in the ForesightGuidelines for Na n o t e c h n o l o gy De ve l o p m e n t. 9th Conference onMolecular Nanotechnology, Santa Clara, CA, Foresight Institute.[5] Anton, P.S., R. Silberglitt, et al. (2001). The Global TechnologyRe volution: Bi o / Na n o / Materials Trends and their Synergies withInformation Technology by 2015 . Santa Monica, CA, RAND.[6] Ibid.[7] Bishop, D., P. Gammel, et al. (2001). “The Little Machines Thatare Making it Big.” Physics Today 54(10): 38-46.[8] Ku rz weil, R. (1999). The Age of Spiritual Machines: W h e nComputers Exceed Human Intelligence. New York, Penguin Books.[9] On the surface this may seem to be a preposterous claim, but there

is significant technological theory to back it up. For further informa-tion compare www. k u rz weilai.net and http://www. t r a n s h u m a n i s t .com/volume1/moravec.htm for starters.[10] Birge, R.R. (2001). Pro t e i n - Based Memories & As s o c i a t i veProcessors. 9th Foresight Conference on Molecular Nanotechnology,Santa Clara, CA, Foresight Institute.[11] Marek, J.J. (2001). The Business of Nanotechnolog y. 2001 ASMEInternational Mechanical Engineering Congress & Exposition, NewYork City, NY, The American Society of Mechanical Engineers.[12] Kurzweil, R. (2001). The Singularity Is Near: A Book Precis,Kurzweilai.net. 2001.[13] Hammond, G.T. (2001). The Mind of War: John Boyd and Amer-ican Security. Washington and London, Smithsonian Institution Press.[14] Army Research Office (2001). Nanoscience for the SoldierWorkshop, Army Research Office. 2001.

Colonel Kip Nygren is Professor and Head of the Department of Civil & Mechanical Engineering at the US Military Academy,

a position he has held since 1995, having served in other positions on the faculty since 1987. He earned a doctorate in

Aerospace Engineering at Georgia Tech in 1986. Earlier his military career consisted of Armor and Aviation assignments in

Europe, Korea and the US and an acquisition assignment in Aviations Systems Command. His current research interests include

the interaction of technology and society, the history of technology and engineering education.

Introduction: Nanoengineered Materials

Materials and material development are fundamental to ourvery culture. We even ascribe major historical periods of oursociety to materials such as the stone age, bronze age, iron age,steel age (industrial revolution), silicon age and silica age (tele-com revolution). This reflects how important materials are tous. We have and always will strive to understand and modifythe world around us and the stuff of which it is made. The nextsocietal frontiers will be opened not through understanding aparticular material, but rather by understanding and optimiz-ing the relative contributions afforded by material combina-tions.

The nanoscale, and associated excitement surro u n d i n gnanoscience and technology (NST), affords unique opportuni-ties to create revolutionary material combinations. These newmaterials will enable the circumvention of classic material per-formance trade-offs by accessing new properties and exploitingunique synergism between materials that only occur when thelength-scale of morphology and the fundamental physics asso-

ciated with a property coincide, i.e. on the nanoscale! The con-fluence of fundamental understanding of materials at this scaleand the realization of fabrication and processing techniquesthat provide simultaneous structural control on the nano-, aswell as micro- and macro-, length scales is the core of the excit-ing area of nanoengineered materials. Examples of such materialtechnologies are rapidly increasing, impacting many diverseareas of the commercial and military arena.

One of the ways nanoscience has advanced the state-of-the-art has been to enhance and improve the properties of existingconventional classes of materials. Polymer composites, forexample, have been a mainstay of high-performance aircraft forover a quarter century, offering a multitude of desirable (andtailorable) properties, such as high strength and stiffness, anddimensional and thermal stability. With the advent and appli-cation of nanotechnology, polymer composites could becomeeven more attractive. As surely as polymer composites changedthe face of industry twenty-five years ago, polymer nanocom-posites will usher in a new era in materials development.

Polymer Nanocomposites Opportunities

The reinforcement of polymers using fillers, whetherinorganic or organic, is common in the production ofmodern plastics. Polymeric nanocomposites, (PNCs) orthe more inclusive term –polymer nanostru c t u re dmaterials, represent a radical alternative to these con-ventional filled polymers or polymer blends. In contrastto conventional systems where the reinforcement is onthe order of microns, PNCs are exemplified by discreteconstituents on the order of a few nanometers -~10,000-times finer than a human hair. The relatives i ze difference between conventional fillers andnanoscale fillers is illustrated in Figure 1.

Uniform dispersion of these nanoscopically-size dfiller particles (or nanoelements) produces an ultra-largeinterfacial area per unit volume between the nanoele-ment and host polymer. For example, interfacial areaapproaching 700 m2/cm3 occur in dispersions of layeredsilicates in polymers. This is comparable to a footballfield within a raindrop! These dimensions also implythat the distance between nanoelements is comparableto their size. For a 1nm-thick plate, the distancebetween plates approaches 10 nm at only 7 vol% ofplates. This is a morphology truly dominated bynanoscale features.

Figure 1. Scanning Electron Micrograph of

Conventional Graphite Flake Filler In An

Elastomer Matrix. The Inset Shows a Scanning

Electron Micrograph of a Collection of ~500

Multi-wall Carbon Nanotubes (MWNT) at the

Same Magnification


Dr. Richard A. VaiaMaterials and Manufacturing Directorate

Air Force Research Laboratory

10µm

This immense internal interfacial area and the nanoscopicdimensions between nanoelements fundamentally differentiatePNCs from traditional composites and filled plastics. Thesecharacteristics imply the overall performance of PNCs can notbe understood by simple scaling arguments that begin with thebehavior of traditional polymer composites. Thus new combi-nations of properties derived from the nanoscale structure ofPNCs provide opportunities to circumvent traditional per-formance trade-offs associated with conventional reinforcedplastics, epitomizing the promise of nanoengineered materials.

From both a commercial and military perspective, the valueof PNC technology is not based solely on mechanical enhance-ments of the neat resin. Rather, its value comes from providingvalue-added properties not present in the neat resin, withoutsacrificing its inherent processibility andmechanical properties. Traditionally, blend orcomposite attempts at ‘multifunctional’ mate-rials require a trade-off between desired per-formance, mechanical pro p e rties, cost andprocessibility. The literature is full of examplesthat suggest PNC technology provides a routearound these traditional limitations [1-6].

For example, consider the rapid advance ofp o l y m e r - l a ye red silicate nanocomposite(PLSN) technology. Efforts within the last 10years have demonstrated a doubling of thetensile modulus and strength without sacrific-ing impact resistance for numerous thermo-plastic (nylon and thermoplastic olefinTPO) and thermoset (urethane, siloxaneand epoxy) resins through the addition of aslittle as 2 vol% layered silicate. Recently,General Motors and partners Basell, Southern

Clay Products and Black-hawk Automotive Plasticsannounced external auto-motive body parts madef rom thermoplastic olefin-layered silicate nanocom-posites. A TPO nano-composite with as little as2.5% layered silicate is asstiff and much lighterthan parts with 10 timesthe amount of conven-tional talc filler. T h eweight savings can reach20 percent depending onthe part and the materialthat is being replaced by

the TPO nanocomposite. On a volume basis, the nanocom-posite parts cost about as much as conventional TPOs becauseless material is needed to manufacture them and no new tool-ing is required to mold the parts. Overall, the weight advantagecould have significant impact on environmental concerns andmaterial recyclibility. For example, it has been reported thatwidespread use of PLSNs by US vehicle manufacturers couldsave 1.5 billion liters of gasoline over the life of one year’s pro-duction of vehicles and reduce related carbon dioxide emissionsby more than 10 billion pounds [6].

Considering the plurality of potential nanoelements, poly-meric resins and applications, the field of PNCs is immense.De velopment of multi-component materials, whethermicroscale or nanoscale, must simultaneously balance fourinterdependent areas: constituent-selection, fabrication, pro-cessing, and performance. For PNCs, this is still in its infancy,and ultimately many perspectives will develop, dictated by thefinal application of the specific PNC. Two main fabricationmethodologies have been developed for PNCs: in-situ routesand exfoliation. Currently, exfoliation of layered silicates and

Figure 2. 7nm Ag Nanoparticles in Kapton

Figure 3. Ag-polybenoxazole Fiber

Figure 4. Liquid Crystal Nanodroplets in a Lamella Morphology


200nm

150nm1500nm

2µm

Figure 6. Nylon

6-Layered Silicate

Nanocomposite

200nm


carbon nanofibers / nanotubes in commodity and high per-formance resins are the most heavily investigated PNCs byindustrial, government and academic institutions world-wideand will be further discussed below. Some examples of pre-commercial PNCs fabricated via in-situ methodologies areprovided in Figures 2, 3 and 4.

In-situ PNCs

In-situ routes to PNCs are based on the creation of thenanoelement within the polymer matrix by chemical means orphase separation. The polymer matrix provides the templatewithin which the nanoelement is formed. An example of thisroute is the decomposition or chemical reaction of a precursor

introduced to the polymer matrix. Figure 2 depicts 7 nm-Agnanoparticles formed in the near-surface region of Kapton viaa solution infiltration approach that is amenable to microscalepattern formation [7].

Figure 3 depicts a Ag-polybenoxazole nanostructured fiberfabricated via a comparable approach [8]. These fibers are ultra-tough, electrically conductive, ~200-times stronger and ~50%lighter than current aerospace signal wire cores. Another exam-ple of the in-situ methodology is chemical or processing perturbation of an initially homogenous polymer system com-prised of a blend of two or more chemically discrete con-stituents to induce a controllable nanoscale phase separation of the constituents.

Figure 7. Comparison of

Traditional Filled (bottom)

vs Nanocomposite (top)

Epoxy Optical Properties

Figure 5. Crystal Structure

of 2:1 Layered Silicates

(Smectites) [10]

δ −

Exchangeable Cations

(Li+, Na+, K+, Ca+)Gallery,

Interlayer

Tetrahedral layer

Octahedral layer

Tetrahedral layer

Silicate Layer

0.96 nm

O Al, Mg, Fe

Si OH

δ +


Figure 4 depicts ~70 nm liquid crystal droplets organized with-in a lamella morphology with ~200nm layer spacing formed viaholographically-induced liquid crystal phase separation from aphotopolymerizable syrup at two magnifications. These struc-tures result in ultrafast (ms) switchable diffractive optics and fil-ters that have military applications. Similar behavior has beendemonstrated for patterning nanoparticles in photopolymeriz-able syrups [9]. The interested reader is encouraged to examinethe associated references [7-9] and explore related efforts in theliterature to disperse, arrange and/or in-situ form quantumdots, oxide nanoparticles, metallic nanoparticles, nanowires,proteins, enzymes, etc. in polymer hosts.

Layered Silicates

Layered silicates (alternatively referred to as 2:1 layered alumi-nosilicates, phyllosilicates, clay minerals and smectites) are themost commonly used inorganic nanoelements in PNC researchto-date. Layered silicates possess the same structural character-istics as the well-known minerals talc and mica and are com-

prised of hydrated aluminosilicate [10]. Their crystal structureand habit are summarized in Figure 5.

In the figure, the Van der Waals interlayer (or gallery) con-taining charge-compensating cations (M+) separates covalentlybonded oxide layers, 0.96 nm thick, formed by fusing two sili-ca tetrahedral sheets with an edge-shared octahedral sheet ofeither alumina or magnesia. The charge per unit cell (generallybetween 0.5 and 1.3) originates from isomorphous substitutionwithin the silicate layer (e.g. tetrahedral Si4+ by Al3+ or octahe-dral Al3+ by Mg2+). The number of exchangeable interlayercations is also referred to as the cation exchange capacity (CEC).This is generally expressed as milliequivalents/100g, and rangesbetween 60 and 120 for relevant smectites. Variation in theamount, type and crystallographic origin of the excess layercharge results in a large family of natural (e.g. montmorillonite,hectorite, saponite) and synthetic (e.g. Laponite, fluorohec-torite) layered silicates exhibiting different physical and chemi-cal characteristics, such as layer size, stacking perfection, reac-tivity and Lewis acidity. For example the lateral dimension ofthe layers range from 20 to 1000 nm.

When exfoliated, an individual sheet is 1 nm thick, withaspect ratios (diameter:thickness) in excess of 100.Conceptually, the process of layered silicate dispersion in poly-mers can be likened to removing, and then arranging, millionsof sheets of paper from thousands of tomes on library shelves touniformly occupy all the free space in the library. The highlyanisotropic structure of the silicate layer is critical to providepercolative behavior at low volume fractions, which manifestsin numerous physical property increases at small amounts ofdispersed layers. Figure 6 depicts the morphology of a Nylon 6-layered silicate nanocomposite [11]. Dark lines are individuallayers of aluminosilicate, 1 nm thick, oriented perpendicular tothe sample surface. This structure results in enhanced mechan-ical properties at elevated temperatures, opening new opportu-nities for plastics in the automotive industry [6], as well asreduced water and CO2 permeability, enabling improved foodand beverage packaging [4].

In general, nanoscale dispersion of the laye red silicateplatelets in resins produces glassy modulus enhancements ofone to two times and rubbery modulus increases of 5-20 fold.Related increases in heat distortion temperature and enhancedelevated temperature mechanical properties (strength and mod-ulus) are commonly reported. Additionally, reduced thermalexpansivity (CTE), matrix swellability, gaseous permeabilityand flammability upon 1-5 vol% addition of exfoliated layeredsilicates will provide new opportunities for various resin sys-tems. An abridged list of polymers investigated with layered sil-icate includes polystyrene, various polyamides (6, 6-6, 11, 12,MDX), polyimides, polypropylene, ethylene vinyl acetatecopolymers, poly(styrene-b-butadiene) copolymers (SBS), elas-tomers (PDMS, EPDM, NBR), polyurethanes, poly(ethyleneoxide), poly(vinyl alcohol), polyaniline, epoxies, and phenolics.Alexandre et al. [5], Beal and Pinnavaia [2], and Krishnamoortiand Vaia [3] provide summaries of some of the polymers andfabrication routes available in the open literature. The bottomline is that these unique property combinations are observed

Figure 8. Demonstration

of PLSN Self-passivation

Behavior

Figure 8a

Char Layer

Virgin Material 200µm

Figure 8b


with surprisingly low volume fraction additions of layered sili-cate (1-5%), thus maintaining polymeric processibility andreducing weight with respect to micron-scale counterparts(>20% loading). Note that these comparisons are with regardto conventional filled polymers and not with regard to contin-uous fiber re i n f o rced composites. PNCs may prov i d eenhanced, multi-functional matrix resins, but should not beconstrued as a potential replacement for current state-of-the-artcarbon-fiber reinforced composites.

From the military perspective, these enhanced polymer sys-tems provide opportunities in many venues. In addition to ageneral desire for higher-performance, higher use-temperatureresins and thermoplastics, PLSNs provide opportunities toaddress material limitations in advanced system concepts.Combinations of reduced impact resistance, CTE control, sup-pression of microcracking and increased modulus are currentlybeing examined for next-generation nanoscale rigid-particulatereinforced resins for advanced fiber reinforced polymeric com-posites in unmanned aerial vehicles [12]. Thermal stability andenhanced fire retardancy through char formation have motivat-ed investigation of PLSNs as a component to anti-flammabili-ty additives for aircraft interiors [13]. Superior barrier proper-ties against gas and vapor transmission have resulted in appli-cations for food and beverage packaging for military rations[14] and for barrier liners in storage tanks and fuel lines forcryogenic fuels in aerospace systems. Also, depending on thetype of polymeric host, PLSNs display interesting ionic con-ductivity for solid-state electrolytes in batteries [15]. Finally,along with enhanced scratch resistance and ballistic perform-ance, the nanoscopic phase dimensions and low volume frac-tions enable maintenance of optical clarity, critical for scratch-

resistant face shields. Figure 7 compares the optical propertiesof traditional filled and nanocomposite epoxy. In the figure theoptical clarity of exfoliated nanoscale morphology (top) is sub-stantially better than the intercalated, conventional filler mor-phology at comparable loadings (bottom). Recent efforts havebegun to explore use of layered silicates as ceramic precursorsthat will react in-situ with aggressive environments to form atough ceramic passivation layer on the polymer surf a c e .Examples of such self-passivation/self healing during exposureto a solid-rocket motor exhaust (top) and plasma environments(bottom) [16] are shown in Figure 8. Combinations of the self-passivation response with CTE control provide unique alterna-tives to current materials being examined for inflatable mem-branes for space antennas and solar collectors. Survivability inreal space environments is currently being examined for PLSNsand other polymer nanocomposites through an exposure exper-iment on-board the International Space Station [17].

Carbon Nanotubes

In contrast to the 25+ year investigation of layered silicate dis-persion in polymers, exploration of carbon nanotubes disper-sion is relatively new, hindered until recently by the limiteda vailability of nanotubes. Howe ve r, nanotube-polymernanocomposites are garnishing substantial attention todaybecause carbon nanotubes offer opportunities to impart uniqueelectrical and thermal properties to the polymer resin as well asto enhance mechanical and physical response. Figure 9 brieflysummarizes the structural aspects of carbon nanotubes [18].Tube diameters can range from 1-100 nm with aspect ratios(length : diameter) in excess of 100 or 1000! As with the lay-ered silicates, the highly anisotropic nature of the tube is criti-

Figure 9. Images of Single-

and Multi-walled Carbon

Nanotubes

Figure 9a Figure 9b

Figure 9c

Figure 9d

TEM cross section view of MWNT.

Tube wall

Tube Axis

Epoxy

3nm

Tube walls

Tube core

100nm


cal to provide percolative behavior at low volume fractions,resulting in graphite-like electrical and thermal properties at 1-2 vol% additions.

Overall, carbon nanotubes (i.e. nanoscopically hollow fibers)are generally considered in two categories: single wall nan-otubes (SWNT) and multi-wall nanotubes (MWNT). SWNTsare comprised of an individual graphene sheet (Figure 9a). Theindividual tubes nominally aggregate into ‘ropes’ consisting offace-center packed individual tubes. Figure 9b shows a trans-mission electron micrograph (TEM) of a random mat of theseSWNT ropes (Figures 9a and 9b courtesy of R. Smalley,cnt.rice.edu/pics.html.) Substantial technological excitementsurrounds single-wall carbon nanotubes, based on theoreticalprediction and initial experimental verification of incrediblemechanical properties (E ~1 TPa [19]) and metal-like electricalconductivity [18] of individual tubes. MWNTs are comprisedof nested single-wall tubes (Figure 9c). The external surface ofa MWNT is a graphene plane and the individual graphenesheets within the tube interact via van der Waals forces.Alternative gaseous MWNT fabrication routes also produce abamboo-like structure (Figure 9d) comprised of short segmentscontaining nested tubes with a finite orientation with respect tothe overall MWNT axis (c o u rtesy Applied Sciences, In c .) .Terminal strained graphene sheets decorate the surface of thesetubes, providing chemical sites for facile surface functionaliza-tion and interfacial tailorability.

Challenges associated with the development of low-costmanufacturing approaches, the processibility of raw SWNTproducts and the maintenance of the SWNT properties duringnanotube-polymer fabrication and processing is limiting rapidsuccess of SWNT as polymer nanofillers. Current prices rangefrom $500-$1000 per gram for purified SWNTs. Nonetheless,substantial academic and government investigations are cur-rently addressing these issues.

In contrast, the commercial availability of multiwall nan-otubes (MWNT) at $60-$100 per pound is facilitating excitingsuccesses today. Motivated by similar product considerations asdiscussed for the PLSN systems (provide comparable mechani-cal properties as achieved with microscale fillers, but at lower

loadings, while maintaining processibility and delivering addi-tional functionality), numerous pre-commercial successes havebeen reported. For example, MWNTs have been used as directreplacements to carbon-black in paint powders, providing suf-ficient electrical conductivity at an order of magnitude lowerfiller loading to enable electrostatic powder coating of automo-tive parts (e.g. Hyperion Catalysis Int.). The ability to chemi-cally tailor the external surface of the tube while maintainingcomplete graphene planes within the tube core provides sub-stantial flexibility to independently tailor mechanical, electricaland interfacial properties. A potential drawback to MWNTsrelative to SWNTs is decreased mechanical reinforcementbecause of the weak bonding between the graphene planeswithin the tube. However, current applications driving nan-otube reinforced plastics appear to be motivated more by addi-tion of electrical and thermal properties than by substantial(10-100x) mechanical property increases, and thus MWNTsare expected to make substantial contributions in this arena.Figure 10 shows homogeneously dispersed carbon MWNTs inan elastomer matrix, providing a conductive plastic with elas-tomeric extensibility in excess of 500% [20].

From the military perspective, tailoring electrical or thermalconductivity of polymer films, fibers or monoliths with verylow loadings provides revolutionary opportunities, spanningfrom electro-magnetic shielding to thermally conductive com-posites to electrostatic discharge layers to smart fabrics for thefuture solider. Furthermore, since these nanocomposites can betailored to be responsive to applied electrical fields, a range ofadaptive structures is envisioned. Even more revolutionary isthe concept of using the nanoscale percolative network of tubesto controllably re m ove or transport electrical charge tonanoscopically structured polymer regions between the tubes,providing the foundation for technologies ranging from next-generation flexible photovoltaics to conformal antennas toenergy storage devices. The potential impact is pervasive, evengreater than that envisioned for PLSNs.

Challenges

Notwithstanding the considerable advances, excitement andpromise of exfoliated PNCs, substantial fundamental researchis still necessary to provide a basic understanding of these mate-rials to enable full exploitation of their nanoengineering poten-tial. Despite the large number of combinations of matrices andpotential reinforcing nanoelements with different chemistry,size, shape and properties, all PNCs share common featureswith regard to fabrication methodologies, processing, morphol-ogy characterization and fundamental physics.

The objective of PNC fabrication via exfoliation methodolo-gy is to uniformly disperse and distribute the inorganic filler,initially comprised of aggregates of the nanoparticles, within thep o l y m e r. The final PNC stru c t u re results from the transforma-tion of the initially microscopic heterogeneous system to ananoscopically homogenous system. In general, four appro a c h-es have been developed to fabricate PNCs via exfoliation - s o l u-tion processing, mesophase mediated processing, in-situ polymeriza-

Figure 10. Scanning Electron Micrograph

of MWNT Dispersion In Silicone Matrix [20]


tion a n d melt pro c e s s i n g . Each methodology has advantages withrespect to the processing steps necessitated by the desired finalform of the PNC (powd e r, film, paste, fiber, bulk monolith).Substantial re s e a rch efforts currently endeavor to address thefundamental challenge of providing general guidelines, includ-ing thermodynamic, kinetic and rheological considerations formorphology control via these fabrication processes. For exam-ple, many potential military opportunities depend on successfulincorporation of the nanoelements in thermoset resins. Su rf a c efunctionalization must be carefully chosen to control polymer-ization rates and initiation points such that separation of thea g g regate of nanoelements occurs before or during polymeriza-tion. This is because the extent of cross-linking reactions deter-mines the gel-point of the matrix, and ultimately the extent ofexfoliation, hence the final PNC morphology [21]. Ad d i t i o n a lcomplications arise from pre f e rential partitioning of the va r i o u schemical components in these multicomponent resin systems( re a c t i ve oligomer, pre p o l y m e r, cro s s - l i n k e r, catalyst, etc.,) to thenanoelement surface modifying reactivities and cre a t i n gu n k n own gradients in network topology.

The key to any of these fabrication processes is the engineer-ing of the polymer-nanoparticle interface. Surfactants are com-monly used to facilitate this process. These range from smallmolecules ionically associated with the nanoparticle surface,such as with layered silicates, to chemically bound small mole-cules or physi-absorbed polymers for nanotubes. These surfacemodifiers mediate interlayer interactions by effectively loweringthe interfacial free energy. Furthermore, they may serve to cat-alyze interfacial interactions, initiate polymerizations, or serveas anchoring points for the matrix and thereby improve thestrength of the interface between the polymer and inorganicfiller. However, choice of the optimal modifier to date is at bestempirical. Most efforts currently focus on developing interfa-cial tailoring that achieves dispersion. This is done withoutregard to providing the necessary thermal stability and desiredinterfacial response for final form processing, or without a pri-ori determination of the desired interfacial characteristics (e.g.strong, intermediate or weak bonding) to maximize materialperformance. However, these later considerations are para-mount to providing components that have a reliable service life.

Developing an understanding of the characteristics of thisinterphase region, its dependence on nanoelement surfacechemistry, the relative arrangement of constituents, and ulti-mately its relationship to the PNC properties is a currentresearch frontier in nanocomposites. Equally important is thedevelopment of a general understanding of the morphology-property relationships for mechanical, barrier and thermalresponse of these systems. This necessitates determining thecritical length and temporal scale at which the continuumdescription of a physical process gives way to mesoscopic andatomistic views of these nanoscale systems. This is one of thecurrent challenges for the burgeoning field of computationalmaterials science (see AMPTIAC newsletters Vol. 5, #2 andVol. 5, #3 for extensive information on the topic ofComputational Materials Science).

Summary

Polymer nanocomposites is a rapidly growing area of nano-engineered materials, providing lighter weight alternatives toconventional, filled plastics with additional functionality asso-ciated with nanoscale specific, value-added properties. If thepromise and excitement surrounding layered silicates and car-bon nanotubes are any indication, the future of PNC technol-ogy is truly boundless. The opportunities to extend PNC con-cepts to other nanoelements and polymer hosts are immense,opening the way to provide tailor-made materials that circum-vent current limitations and enable future concepts for theDOD and the defense community at large.Acknowledgements

The current article is based on previous collaborations with E.P.Giannelis and T. Benson-Tolle. M. Alexander (Figures 1,8), E.Manias (Figure 3) and T Bunning (Figure 2c) contributed tothe Figures. The Air Force Research Laboratory, Materials andManufacturing Directorate provided financial support.

References

[1] E.P. Giannelis, Adv. Mater. 8, 29 (1996).[2] Polymer Clay Nanocomposites, T.J. Pinniavaia, G.W. Bealeds. 2000.[3] Polymer Nanocomposites, R.A. Vaia, R. Kishnamoorti,eds., American Chemical Society, Vol 804, Washington, DC2001.[4] Nanocomposites 1999: Polymer Technology for the NextCentury. Principia Partners, Exton, PA 1999.[5] M. Alexandre, P. Dubois, Mater. Sci. Eng. 28, 2000, 1-63.[6] Nanocomposites: New Low-Cost, High-Strength Materialsfor Automotive Parts, National Insititutes of Technology, ATPProject, 97-02-0047, 1997.[7] R.A. Vaia, J-W. Lee, B. Click, G. Price, C.S. Wang inOrganic-Inorganic Hybrid Materials, R.M. Laine, C Sanchez,C.J. Brinker E.P. Giannelis eds. MRS Proceedings, vol 519,Pittsburgh, PA 1998, 201-209.[8] R.A. Vaia, J-W. Lee, C-S. Wang, B. Click, G. Price, Chem.Mater. 1998, 10, 2030.[9] R.T. Pogue, R.L. Sutherland, M.G. Schmitt, L.V.Natarajan, S.A. Siwecki, V.P. Tondiglia, T.J. Bunning, AppliedSpectroscopy, 54(1), 12-28A, 2000. R.A. Vaia, C.L. Dennis,L.V. Natarajan, V.P. Tondiglia, D. Tomlin, T.J. Bunning, Adv.Mater. 13, 2001, 1570-1574.[10] Grim, R.E. Clay Minerology, 2nd ed. McGraw Hill, NewYork, 1968, p. 362.[11] A. Usuki, M. Kawasumi, Y. Kojima, A. Okada, T.Kurauchi, O. Kamigaito, J. Mater. Res. 8, 1174-1183 (1993).Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, T. Kurauchi, O.Kamigaito, J. Polym. Sci.Part A: Polym. Chem. 31, 983 (1993).[12] C. Chen, D. Curliss, SAMPE Journal, 37(5), 7-18, 2001.[13] J.W. Gilman, Applied Clay Sci. 15, 1999, 31-49.[14] G.W. Beall in Polymer-Clay Composites, T.J. Pinniavaiaand G.W. Beall eds, John Wiley and Sons, New York, 2001, p.267. Yano, K. Usuki, A.; Okada, A. J. Poly. Sci. A: Polym.Chem. 35, 1997, 2289-2294.


[15] E. Ruiz-Hitzky, P. Aranda, B. Casal, J.C. Galvan, Adv.Mater. 7, 1995, 180. Vaia, R.A., Vasudevan, S., Krawiec, W.;Scanlon, L.G.; Giannelis, E.P. Adv.Mater. 7, 1995, 154-156.[16] R.A. Vaia, G. Price, P.N. Ruth, H.T. Nguyen, J.Lichtenhan, App. Clay Sci. 15, 1999, 67-92. H. Fong, R.A.Vaia, J.H. Sanders, D. Lincoln, Chem. Mater, 2001, 13, 4123-4129.[17] Materials International Space Station Ex p e r i m e n t ,MISSE, FS-2001-07-136-MSFC, ISS Mission 7A.1, STS-105.[18] R. Saito, G. Dresslhaus, M.S. Dresselhaus, Physical

Pro p e rties of Carbon Nanotubes, Imperial College Pre s s ,London.[19] B.T. Kelly, Physics of Graphite (Applied Sci, London,1987). Yu, Files, Arepalli, Ruoff, Phys. Rev. Lett, 84, 2000.[20] M. Alexander, C.-S. Wang in Proceeding of 2nd AnnualNa n o s t ru c t rued Materials 2001: Application andCommercialization, June 2001, Chicago.[21] T. Lan, P.D. Kaviratna, and T.J. Pinnavaia, Chem. Mater.,7, 2144 (1995). T. Lan and T.J. Pinnavaia, Chem. Mater., 6,2216 (1994).

Dr. Richard Vaia is currently a senior materials scientist in the Air Force Research Laboratory. Additionally, he is Research

Coordinator for Nanotechnology in the US Air Force’s Materials and Manufacturing Directorate (AFRL/ML) and Direction Leader

for Polymeric Photonics in the Polymer Core Technology Area. Rich is an authority on polymer/inorganic nanocomposites and

his research group focuses on 1) understanding processing-structure-property relationships of polymer-inorganic nanostructured

system for use in space and propulsion technologies; and 2) fabrication of complex core-shell nanostructures and development

of techniques to achieve three-dimensional control of particle arrangement on meso, micro and millimeter scales for photonic

systems.

Dr. Vaia received a Ph.D. from Cornell University (Dr. Emmanuel Giannelis, Materials Science and Engineering) in 1995,

following a M.S. and B.S. in Materials Science from the Materials and Engineering Department at Cornell. He currently holds

3 patents, is the editor of one book, and has published over 40 peer-reviewed journal articles in nanomaterials and nano-

technology.


Introduction

The use of nanomaterials for energy storage and conversion inelectrochemical power sources is a rapidly growing field withtremendous potential. Even though the use of nanomaterials inpower sources is not new, the science at the nanoscale has stillnot been fully exploited. A prime example is the use of highlydispersed nanoscale platinum particles (and more recentlyalloys or mixed-metal systems) as effective electrocatalysts infuel cells. Although fuel cells have used nanoscale platinum cat-alysts for decades, their performance improves yearly as scien-tists learn to control and modify the nanoscale catalysts in theelectrode structures [1]. Due to advances in analytical instru-mentation and modeling, the complex electrocatalytic process-es in fuel cells, which require concurrent electronic and ionict r a n s p o rt with simultaneous catalytic activation at thenanoscale, are beginning to be addressed in a scientific manner,as opposed to previous empirical approaches.

The challenges and opportunities for nanoscience in powersources lie in the understanding, control, and fabrication ofcomplex structures and composites at the nanoscale. Only afterengineers and scientists have gained a strong footing in theseareas will it be possible to fully realize and exploit the capabili-ties of this technology. Certain phenomena are only possible onthe nanoscale due to the dominance of interfacial physics andchemistry at large surface-to-volume ratios, such as the quan-tum interactions between nanoscale material phases. By learn-ing to control the physics and chemistry at these small lengthscales, one can envision nanoarchitectured materials engineeredto exhibit properties and performance that far exceed the “sumof the parts.” One such revolutionary approach would be theimplementation of an array of individual electroactive nanoar-chitectures performing ultra-fast computations, using single-electron “bits” shuttled between a nanoscale electron storagesource and driven by nanoscale electrochemical voltage sources.

Figure 1. A Conceptual Nanoarchitectured, Integrated

Battery-computer with Electro-optic Interfaces to the Macro-world

Dr. Richard T. CarlinDirector of the Mechanics and Energy Conversion Division

Office of Naval Research

Dr. Karen Swider-LyonsChemistry Division

Naval Research Laboratory

Cathode “Sink”

Quantum Dot

Anode “Source”

“Gate”

Nanoscale Electro-optic

Interface to Cathode

Nanoarchitectured Array: Anode/Cathode

and Computational Element

Nanoscale Electro-optic

Interface to Anode

I/O Interface to Macro-World

26

Nanoarchitectured electro-optic devices could be used forinternal interconnects and to connect the array to them i c roscale world for data input-output and intermittentrecharging. Such a complex nanoarchitecture would remove thelimitations of electron transport between functioning nanoscalecomponents. The concept is illustrated in Figure 1.

The potential revolutionary benefits of nanoarchitecturedpower-source materials, composites, and eventually multifunc-tional structures, make them prime strategic research areas. Toexploit these potential benefits, the Office of Naval Research(ONR) and the Na val Re s e a rch Laboratory (NRL) haveembarked on the challenging path to develop these concepts.This article provides examples of research directed towardunderstanding, controlling, and fabricating complex structuresand composites for battery and fuel cell applications. Suchresearch is only in its infancy and is likely to undergo dramaticevolutionary and revolutionary changes as insight into nanoar-chitectures becomes clearer through scientific investigations inall research areas. While the examples presented here come pri-marily from work supported by ONR, a tremendous amountof similar and related research is ongoing around the world andis introduced where appropriate. Recent insightful overviewson electrochemical applications of nanomaterials have

appeared, including topics on aerogels [2], nanostructured bat-tery materials [3], production of nanostructured electroceram-ics [4], and nanostructured solid-state electrolytes [5].

Batteries

Battery cells are fundamentally composed of three active com-ponents: an anode (negative electrode), a cathode (positive elec-trode), and an electrolyte. The functions of the anode and cath-ode are to store, release, and reincorporate electrons andcharge-balancing ions during discharge and charge cycles, thusthe electrodes must possess both high ionic and electronic con-ductivities. The amount of electrical charge stored by an elec-trode is referred to as the electrical capacity, typically expressedeither in units of amp-hours per kilogram (Ah/kg) or amp-hours per liter (Ah/l). Electrical capacity is multiplied by theelectrochemical potential of the battery to obtain its total avail-able energy on a weight or volume basis, typically expressed inunits of Watt-hours per kilogram (Wh/kg) or Watt-hours perliter (Wh/l) respectively. The electrodes interchange ions in acell through an electronically insulating electrolyte (liquid orsolid) that has high ionic conductivity to minimize internalbattery resistance. Cells are used individually or linked in seriesor parallel to meet the capacity and energy requirements of anexternal load. Additional information on batteries is readilyavailable from many resources [6-9].

The focus of many current battery research and developmentprograms is lithium-ion and lithium-metal batteries [9]. Thecathodes for both these cell chemistries typically contain a tran-sition-metal oxide lithium-insertion compound, such asLiCoO2. The anodes for lithium-ion batteries are usually alithium-carbon insertion compound or a lithium-metal alloy,while metallic lithium is employed as the anode in lithium-metal batteries. Either an organic liquid or a solid-state lithi-um-ion conductor serves as the electrolyte in both lithium-ionand lithium-metal batteries. During operation, lithium ionsshuttle between the electrodes through the electrolyte, whileelectrons are driven through the external electrical circuit by theelectrochemical potential (typically 3 to 4 V). A Lithium-ionbattery being charged is depected in Figure 2.

Lithium Battery Anodes

Lithium metal offers the highest theoretical specific capacity ofany anode material (3,862 Ah/kg), but dendrite formation dur-

Figure 3. High-

resolution TEM

of 1:2 Mole

Ratio Si/TiN

Nanocomposite

Anode

Figure 4. High Li-

capacity Templated

Anode Consisting of

110-nm Diameter

SnO2 Nanofibers

[15]

Figure 2. Illustration of a Lithium-ion Cell

(Figure Courtesy of D. Sadoway and

A. Mayes, Massachusetts Institute of

Technology)

External Charging

Circuit

Anode Electrolyte Cathode

Li+

Li+

Li+

Li+

e- e-

LiMOx Carbon Polymer

Li+

Li+

Li+

Li+

Li-Carbon

Li-Alloys

SnOx/Sn

20nm



ing cycling has thus far prevented its practical use in recharge-able batteries. Dendrite formation can lead to cell shorting andto the formation of small particles of metallic lithium that canundergo detrimental, exothermic reaction with the organicelectrolytes. Lithium alloys are viable options for high-capacityanodes (LiAl alloy: 993 Ah/kg and 2,681 Ah/l based on theweight and volume of Al, respectively); however, most of thedesirable alloying metals undergo large dimensional changesupon lithium alloying that cause fragmentation and rapidcapacity loss during cycling. Most current battery developmenttherefore focuses on lithium-ion chemistries with graphiticanodes having theoretical weight and volume electrical capaci-ties of 372 Ah/kg and 837 Ah/l, respectively.

Recently, researchers have discovered that nanoscale tin (Sn)particles encapsulated in an inert inorganic matrix can undergoreversible lithium alloying over many cycles with minimal loss-es in capacity [10]. Some examples of these nanocompositeanodes include Sn-B0.56P0.4Al0.42O3.6 glass (600 Ah/kg, 2,200Ah/l) [10], Sn2Fe-SnFe3C (620 Ah/kg, 1,600 Ah/l) [11], andSn-Li2O (600 Ah/kg) [12]. The exact role that the nanocom-posite structure plays in anode cycle-life and capacity is not wellunderstand and is an active area of research [3].

With the identification of appropriate nanoscale structures,new nanocomposite materials are being designed for use ashigh-performance lithium-ion anodes. For instance, ProfessorPrashant Kumta’s research group at Carnegie Mellon Universityis fabricating nanoscale composites of silicon (Si) and titanium-nitride (TiN) with high lithium capacities [13]. The Si phasereversibly alloys with lithium to form a high capacity storagemedium, while the TiN phase is non-reactive with lithium, buthas high mechanical strength to prevent cracking, and is a goodelectrical conductor (2-5x104 Ω-1 cm-1). When mixed togetherat the nanoscale, the Si/TiN composite has the electrical andmechanical pro p e rties needed for a high-capacity lithiumanode. A 1:2 mole ratio Si:TiN nanocomposite obtained after12 hours of high energy mechanical milling (HEMM) exhibitsa stable capacity of 300 Ah/kg. This is lower than the theoreti-cal value of 776 Ah/kg, calculated assuming complete reactionof Si with 4.4 molar equivalents of Li; however, the experi-mental volumetric capacity of 1,100 Ah/l exceeds that of a lithi-um graphite electrode by 30%. A high-resolution transmissione l e c t ron micrograph (TEM) of a 1:2 mole ratio Si / Ti N

nanocomposite is shown in Figure 3, in which the dark spotsare electrically conducting TiN nanoparticles of <20 nm diam-eter embedded in an amorphous Li-ion conducting Si phase.From TEM and x-ray diffraction studies, it appears that thenanocomposite consists of nanoscale TiN particles embeddedin an amorphous Si phase. Ongoing work intends to increasethe lithium capacity and overall performance of the Si/TiNanode using various strategies, including determining and elim-inating the sources of interfacial resistances between the Si andTiN nanoparticles.

Another promising developmental approach to nanoarchi-tectured lithium-alloy anodes is to use templated nanostruc-tures to fabricate nanoscale materials having the specific sizesand dimensions needed for optimum performance. This workwas pioneered by Professor Charles Martin (University ofFlorida, Gainesville) with support from ONR [14]. One exam-ple is a templated anode consisting of 110-nm-diameter SnO2

nanofibers (shown in Figure 4) irreversibly reduced to a Sn-based nanocomposite. This anode is capable of 800 reversiblecharge-discharge cycles with capacities exceeding 700 Ah/kg[15]. In addition, discharge rates are an order of magnitudeg reater than those achieved in conventional lithium-ionanodes, and capacity losses of the nanocomposite anode areminimal at these rapid discharge rates. The performance ofthese nanoarchitectured anodes is attributed to the small size ofthe individual nanofibers and to the small domain size of theSn grains within the nanofibers.

Lithium Battery Cathodes

Numerous materials have been studied for use as lithium-metaland lithium-ion battery cathodes, including metal oxides,metal sulfides, conducting polymers, and polysulfides.Transition-metal oxides are the most successful materialsbecause of their chemical and structural stability, high lithium-ion capacity, and favorable electrical properties (such as highconductivity of lithium-ions and electrons). Table 1 summa-

Table 1. Performance of Selected Transition-metal

Oxides as Lithium-ion Battery Cathodes

Cathode Theoretical Practical Average

Material Capacity Capacity Cell Voltage

(Ah/kg) (Ah/kg) (V)

LiCoO2 274 142 3.6

LiNiO2 275 145 3.6

LiNi0.8Co0.2O2 274 180 3.6

LiMn2O4 148 120 3.8

Figure 5. (a) TEM of POEM-b-PMMA

Containing a Precursor

Gold Salt (LiAuCl4)

(b) A Conceptualization of a Highly

Reversible Self-organizing

Nanocomposite

Electrode (SONE)

250nm

Figure 5a Figure 5b

POEM Region with

Au Nanoparticles

PMMA Region


rizes the theoretical and practical performance of state-of-the-art metal oxide cathode materials. The theoretical capacity iscalculated from the total lithium-ion content of the compound,whereas the practical capacity corresponds to the actual amountof lithium that can be reversibly intercalated. (Cell voltagesquoted in Table 1 assume a lithium-graphite anode.) The prac-tical lithium capacity of the oxides is lower than the theoreticallithium capacity, because the metal-oxide structures allow onlya portion of the lithium ions to be reversibly accessed duringelectrochemical cycling.

In addition to the materials listed in Table 1, the lithium-ionintercalation properties of vanadium oxides have attracted agreat deal of interest. The vanadium oxides display a range ofV:O molar ratios (e.g., V2O5, V3O8, and V6O13) and possessgood lithium capacity; however the voltage of the VOx is lower(~2-3 V) than the other metal oxides (~3-4V). Recently,nanophase V2O5 cathode materials have been prepared withcapacities up to 450 Ah/kg and a corresponding specific ener-gy of approximately 1,500 Wh/kg when coupled with a lithi-um metal anode [2, 9]. Other nanophase cathode materials,including some in Table 1, show increased capacities comparedto the same compositions having micron-scale particles. Thishas been attributed to facile Li+ diffusion into the interior of thenanoparticles with a concomitant reduction in electrode polar-ization.

The nanoscale vanadium oxides discussed above have suf-fered from capacity loss during cycling, however, ongoingresearch support by ONR is helping to resolve this problem.Recently, Professor Bruce Dunn’s research group at UCLA pro-duced V2O5 nanocomposites, in which carbon nanotubes areused to electrically “wire” the poorly conducting vanadium-oxide nanoparticles. The benefits are dramatic when thisnanoarchitectured approach is used: the practical capacity ofthe wired vanadium-oxide exceeds 400 Ah/kg, which is 2 to 3times that of current cathode materials, and there are no capac-ity losses observed over 20 cycles at high discharge rates.

More recently, Professor Dunn has demonstrated a highreversible capacity for magnesium ions in the V2O5 cathodes(1.5 Mg2+ per V2O5). The electrochemical intercalation of thelarge Mg2+ ions is not possible with mesoscale vanadium oxides.This is a key discovery because the double-charge of Mg2+ car-ries almost twice the energy of Li+, hence the development ofthis concept may lead to magnesium-based rechargeable batter-ies that offer high capacities and are inherently safer than lithi-um batteries.

Electrolytes

Two successful approaches in the development of nanoarchi-tectured polymer electrolytes for high-capacity lithium-ion bat-teries are to incorporate inorganic nanoparticles into a conven-tional polymer electrolyte (e.g. polyethylene oxide with lithiumsalt) or to create block copolymers that self-organize intonanophase regimes that synergistically provide lithium-ion

conductivity and structural stability.With regard to the former approach,nanoparticle guests, such as LiAlO2,Al2O3, TiO2, and SiO2, improve theperformance of polymer electrolytes.The nanoparticles disrupt the poly-mer-chain organization and crystalliza-tion at the nanoscale and create lithi-um ion conducting pathways by pro-moting Lewis acid–base interactionsb e t ween the inorganic nanopart i c l eand the polymer segments [16]. Theseeffects result in lithium-ion electrolyteswith increased lithium-ion conductivi-ty and an enhanced lithium-ion trans-p o rt number, and in addition, then a n o p a rticles stabilize the lithium-polymer interface [16].

Professors Anne Ma yes andDonald Sadoway at MIT have been

Figure 6. Nanoarchitecured Pt/C-SiO2

Electrocatalysts Prepared using Sol-Gel

Techniques (SCF is supercritical fluid)

Figure 7. “Natural” Nanocomposite of Intertwined RuO2

Nanocrystals and Hydrous Proton-Conduction Regions

Vulcan Carbon 9 wt. % Pt/C +

Pt sol

(2nm)SCF CO2 dry

Silica sol

Pt/C-SiO2

composite

aerogel

RuO2 nanocrystals

hydrous boundaries

2.5 nm


exploiting the elegant nanoarchitectures of block copolymers toimprove the performance of lithium-polymer batteries [17].They employ poly[oligo(oxy-ethylene) methacrylate]-b-poly-(methyl methacrylate), or POEM-b-PMMA, with LiCF3SO3

salt as a polymer electrolyte in a Li/polymer/amorphous VOx

cell to achieve cathode capacities of 400 Ah/kg over 200 cyclesand projected cell energy densities of >350 Wh/kg. These blockcopolymers are effective lithium-ion electrolytes because thehydrophilic oxy-ethylene regions transport lithium ions, whilethe hydrophobic regimes provide structural stability plus a“t r a p” to inhibit transport of hyd rophobic anions (i.e.hydrophobic anions will stay in the hydrophobic regions awayfrom the hydrophilic regions), and thus increase the lithiumion transport number.

The Mayes and Sadoway research groups have also preparedself-organizing nanocomposite electrodes (SONEs) in whichgold nanoparticles are incorporated into the lithium-ion con-ducting regions to serve as highly reversible lithium-gold alloyanodes. The segregation at the nanoscale of the gold-precursorsalt into the POEM region of the block copolymer structure isshown schematically in Figure 5a. The salt becomes segregatedwithin the relatively hydrophilic POEM regions and becomesvisible as the dark regions in the TEM micrograph. Upon elec-trochemical reduction, the gold-precursor salt is reduced tometallic gold nanoparticles, as illustrated in Figure 5b. The goldnanoparticles then can undergo highly reversible charge-dis-charge cycling (>700 cycles). The preparation of SONEs thatincorporate other lithium-alloying nanoparticles, such as alu-minum, tin, and silicon, may lead to the development of low-cost, practical, high-performance nanoarchitectured anodes forlithium-ion polymer batteries.

Fuel Cells

As with batteries, fuel cells are composed of an anode, a cath-ode, and an electrolyte. However, the anode and cathode donot store charge but rather electrocatalytically activate a fuel(e.g., hydrogen) at the anode and an oxidizer (e.g., oxygen) atthe cathode. Effective electrocatalysis at the electrodes requiresconcurrent ionic and electronic transport [1]. Achieving thesesimultaneous dynamic processes requires the intersection ofmultiple phases, including the electrocatalyst (e.g., Pt), an ionconductor (liquid or solid-state), an electronic conductor (e.g.,carbon powder), and the gas. By controlling the intersection ofthese regions at the nanoscale, dramatic improvements in fuel-cell performance can be achieved. In addition, the fuel cell elec-trolyte can be enhanced through nanoscale structural optimiza-tion. In fact, the archetypal proton exchange membrane (PEM)fuel cell electrolyte, Nafion™, is itself a nanostructured mate-rial, possessing nanoscale hydrophilic regions that conduct pro-tons and fluorinated hydrophobic regions that provide struc-tural stability [1].

ELECTROCATALYSIS Ongoing research at NRL has providedvaluable insight into how the design of nanostructured electro-catalytic architectures can improve fuel cell performance. Dr.Debra Rolison and co-workers at NRL have prepared nano-

architectured Pt electrocatalysts in a systematic manner usingsol-gel techniques (illustrated in Figure 6; for additional infor-mation, see reference 18). The Vulcan carbon powder providesa continuous electronic network to the 2-nm carbon-support-ed colloidal Pt nanoparticles within an architecture defined bythe continuous nanoscale network of the SiO2 aerogel. Thecontinuous 3-D mesoporous path provides a flux of fuel to thePt electrocatalyst, resulting in a >10,000x increase in its cat-alytic activity for methanol oxidation. This approach empha-sizes the advantages of designing a multifunctional nanoarchi-tectured site to improve the performance of fuel-cell catalysts.

Ad vancements in analytical methods are also leading toi m p rovements in the development of active nanostru c t u re s .N R L’s Dr. Karen Sw i d e r - Lyons and co-workers in collaborationwith Professor Takeshi Egami and Dr. Wojtek Dmowski at theUn i versity of Pe n n s y l vania have used advanced structural analy-sis methods to demonstrate that the electrochemical pro p e rt i e sof hyd rous ruthenium oxide are due to its innate nanocompos-ite nature. Diffraction methods show that this “n a t u r a l”nanocomposite is composed of nanoscale wires of electro n i c a l l yconducting Ru O2 n a n o c rystals that are surrounded by hyd ro u sp roton-conducting regions [19], as shown in Fi g u re 7. The inti-mate mix of electronic and protonic conduction in thisnanocomposite material explains its high activity as a co-catalystfor carbon monoxide-tolerant Pt - Ru Ox fuel cell electro c a t a l y s t s[1] and is key to understanding the performance of hyd ro u sruthenium oxide as a pseudocapacitor [2]. Understanding therelationship between the nanoscale stru c t u re and electro c h e m i-cal pro p e rties in materials will also lead to the design of othera c t i ve materials for electrochemical power sources.

ELECTROLYTE As mentioned earlier, PEM fuel cells alreadyemploy the nanocomposite electrolyte Nafion™. Perhaps evenmore interesting, however, is the potential for nanoarchitec-tured structures to dramatically enhance the performance of theceramic electrolytes used for high-temperature solid oxide fuelcells. Recent studies on nanocomposites of alternating layers ofCaF2 and BaF2 show a linear increase in ionic conductivity (flu-oride ion) parallel to the interface when the layers are 50 and430 nm thick [20]. Space-charge calculations indicate that theincrease is proportional to the interface density. However, a dis-proportionate increase in conductivity is observed when thealternating layers are narrower (16 to 50 nm thick). Under suchconditions, the interfacial space-charge regions overlap, and thenanoscale layers lose their individual behaviors, taking on thep ro p e rties of a super-ionic-conducting material possessinganomalous transport properties [20]. Similar enhancements inionic transport (and electronic transport) may be possible withother ceramics, opening up exciting opportunities for nano-architecturing of future fuel-cell electrolytes.

Opinions, interpretations, conclusions, and recommendations arethose of the authors and are not necessarily endorsed by the Officeof Naval Research or the Naval Research Laboratory.

30

References

[1] Gottesfeld, S. Advances in Electrochemical Science andEngineering, R.C. Alkire, H. Gerischer, D.M. Kolb, and C.W.Tobia, Editors, Vol. 5, P. 195 (1997).[2] Rolison, D.R. and Dunn, B. J. Mater.Chem. 11, 963(2000).[3] Nazar, L.F., Goward, G., Leroux, F., Duncan, M., Huang,H., Kerr, T., and Gaubicher, J. Int. J. Inorg. Mater, 3, 191–200(2001).[4] Schoonman, J. Solid State Ionics 135, 5 (2000).[5] Tuller, H. J. Electroceramics 1, 211 (1997).[6] Tuck, C.D.S. Modern Battery Technology, Ellis Howard,Chichester (1991).[7] Scrosati, B. and Vincent, C.A. Modern Batteries, Secondedition, Arnold, London (1997).[8] Pistoia, G. Lithium Batteries Elsevier, Amsterdam (1994);Tarascon, J.M. and Armand, M. Nature 414 359 (2001).[9] Winter, M., Besenhard, J.O., Spahr, M.E., and Novák, P.Adv. Mater. 10, 725 (1998).[10] Idota, Y., Kubota, T., Matsufuji, Maekawa, and Miyasaka(1997) Science 276, 1395.[11] Mao, O. and Dahn, J.R. (1999) J. Electrochem. Soc. 146,423.

[12] Foster, D.L., Wolfenstine, Read, J.R., and Behl, W.K.(2000) J. Electrochem. Soc. 3, 203.[13] Kim, I., Kumta, P.M., and Blomgren, G.E. Electrochem.Solid-State Letters, 3, 493 (2000).[14] Hulteen, J.C. and Martin, C.R. J. Mater. Chem . 7, 1075(1997).[15] Li, N., Martin, C.R. Martin, Scrosati, B. J. Power Sources97-98, 240 (2001).[16] Croce, F., Persi, L., Ronci, F., and Scrosati, B. Solid StateIonics 135, 47 (2000).[17] Huang, B., Cook, C.C., Simon, M., Soo, P.P., Staelin,D.H., Mayes, A.M., and Sadoway, D.R., J. Power Sources 97-98, 674 (2001).[18] Anderson, M.L, Stroud, R.M., Rolison, D.R. Nano Lett. 2(2001), in the press.[19] Swider-Lyons, K.E., Bussmann, K.M., Griscom, D.L.,Love, C.T., Rolison, D.R., Dmowski, W., and Egami, T. inSolid State Ionic Devices II - Ceramic Sensors, edited by E. D.Wachsman, et.al, Electrochemical Society Proceedings 2000-32(2000), p 148; Dmowski, W., and Egami, T., Swider-Lyons,K.E., Love, C.T., Rolison, D.R., submitted to J. Phys. Chem.[20] Sata, N., Eberman, K., Maier, E. Nature 408 946 (2000).

Dr. Richard T. Carlin is cur rently the Director of the Mechanics and Energy Conversion Division at the Office of Naval Research

(ONR). Previously he served as an ONR Program Officer for the Undersea Propulsion and Electrochemistry S&T Programs. Other

past positions have included Research Chemist, Air Products & Chemicals, Inc.; Assistant Professor of Inorganic/Analytical

Chemistry, University of Alabama; and Chief of the Electrochemistry Division, Frank J. Seiler Research Laboratories, USAF

Academy. Dr. Carlin holds a Ph.D. in Inorganic Chemistry from Iowa State University and has published over 100 scientific papers

and been awarded 7 US patents.

Dr. Karen Swider-Lyons is a staff scientist in the Chemistry Division at the Naval Research Laboratory in Washington DC. She stud-

ied Chemistry as an undergraduate at Haverford College and earned her Ph.D. in Materials Science and Engineering from the

University of Pennsylvania in 1992. Her research is dedicated to power sources, with a focus on the study of transport mecha-

nisms in solid state ionic materials. She currently does research and development of microbattery systems and in new oxide-based

catalysts for use in polymer exchange membrane fuel cells. She also serves as a technical consultant on two DARPA fuel cell pro-

grams. Dr. Swider-Lyons is a member of the Materials Research Society, the Electrochemical Society, and the International Society

for Solid State Ionics.


AMPTIACAMPTIAC

15M a t e r i a lM a t e r i a lE A S E

A M P T I AC is a DOD In fo rmat ion Anal ys is Center Ad m i n i s te red by the Defense In fo rmat ion Systems Ag e n c y, Defense Te chnical In fo rma ti on Cen ter and Opera ted by I IT Re s e a rc h Ins t i t u te

This article provides a brief overview of the major events, science, tools and processes in nanotechnology. Part One highlights the history of nanotechnology,

Part Two discusses the tools used to study such small-scale materials and the underlying mechanisms responsible for the novel properties observed, and Part Three

covers the major processing methods of nanomaterials.

MATERIALS ENGINEERING WITH NATURE’S BUILDING BLOCKS

Richard Lane, Benjamin Craig and Wade Babcock

AMPTIAC, Rome, NY

17 AMPTIAC

Part One: His tory of Nano Materials Engineering

Nanotechnology is definitely one of the most talked about “new” areas of

study today. The idea that nanostructured materials would exhibit unique prop-

erties is accredited to Gleiter and Turnbull for the work they did independent-

ly in the early 1980s [1]. Since then the field of nanotechnology has expand-

ed dramatically due to the realization of its potential technological benefits.

Nanotechnology basically is the process of manipulating matter at the

atomic scale. There are many interpretations of what this includes, but for our

purposes and our inherent focus on materials engineering, we will include the

purposeful creation, manipulation and machining of engineered materials at

the nanoscale. Typically, nanomaterials have some critical dimension more

than 1 and less than 100 nanometers, where a nanometer is one billionth of

a meter, or one thousandth of a micron. The typical “yardstick” at small scales

- a human hair - is about 200,000 nanometers in diameter. (A more thorough

description of nanoscale is included in Part Two.)

Humans have been utilizing nanomaterials for centuries, dating back to

Roman potters and possibly before. Ancient craftspeople utilized nanoscale

particles in glazes to create unique colors that changed with incident lighting.

In the 19th century, the use of metals in new and astounding structural appli-

cations drove the importance of microstructure to the front of engineers’ minds.

With this focus, empirical methodologies for processing metals yielded mas-

sive improvements in strength, toughness, ductility and hardness. Much of this

work was microtechnology, even before the field existed, and one could argue

that it bordered on the nano regime with the empirical manipulation of

microstructure, precipitates and crystallographic grains.

Eventually, innovations such as the automobile demanded more durable

consumables like rubber, glass, and ultimately, plastics. Rubber may have been

the first modern example of nanotechnology, with the addition of carbon black

and sulfur to improve durability. Even though the ability of engineers to study

the exact mechanisms at work did not exist, carbon black’s nanoscale parti-

cles served to modify the behavior of rubbers, thus assisting the growth of one

of the most important manufacturing booms of the late 19th and early 20th

centuries. While nanotechnology claims its roots in chemistry, the fields of

physics and engineering have joined the fray and are pushing the boundaries

of the possible.

Many trace the roots of modern nanotechnology and nanomaterials to a

1959 talk given by Dr. Richard Feynman at a meeting of the American Physical

Society.* It was in this now-famous lecture that Feynman proposed the all-too-

simple hypothesis that there was “plenty of room at the bottom.” He speculat-

ed that future scientists and engineers would build complex structures from

atoms and molecules.

It wasn’t until 1974 that the field was given the name “nanotechnology”

however, and that is widely attributed to University of Tokyo researcher Norio

Taniguchi. He made the distinction between engineering at the micrometer

scale (the basis for modern microelectronics that was just starting to hit its stride

in the 70s) and the new field of sub-micrometer engineering that was begin-

ning to emerge†[2]. Nascent nanotechnology began to “grow up” and enter

the mainstream consciousness in the mid-1980s with work by Richard Smalley

at Rice University on what would be called Buckminster Fullerenes (now

famously known as “Buckyballs”) and MIT researcher K. Eric Drexler’s publi-

cation of “The Engines of Creation.”

While Smalley tantalized chemists and physicists with the discovery of a

“new” form of one of the building blocks of nature, Drexler outlined a future

dominated by a new form of manufacturing done at the molecular, and even

atomic level. The study of Buckyballs has led to the discovery of tube-like struc-

tures of carbon atoms which are basically sheets of graphite rolled up with

their edges connected to form a cylinder. They can be thousands of times

longer than they are in diameter. It is these carbon nanotubes (CNTs) which

have piqued the interest of so many engineers as they may hold the promise

of extremely high tensile strength inclusions for nanocomposites, structural

beams for nanomachines, and possibly even conductors (wires) for nanoelec-

tronics. CNTs and other molecular-level structures form the bases of what

Drexler describes as a coming revolution in molecular manufacturing.

What is most amazing about the development of micro- and then nan-

otechnology is the pace at which it has advanced. Taniguchi predicted in

1974 that within 15 years there would be machining methods capable of sub-

100 nm dimensional precision. This prediction was largely proven correct. By

the early 1990s, nanotechnology’s capabilities could not only image and

probe atomic structures, but move individual atoms, one at a time, around on

a substrate. The “quantum stadium” image generated by IBM researchers pro-

vides a poignant example of this in Figure 1. The image consists of 76 iron

atoms on a copper substrate. Wave patterns in the interior are from the densi -

ty distribution of trapped electrons.

Since 1990, significant advancements have been made, both in actual

materials fabrication and in computational simulation. The manufacturing of

CNTs and many other nanomaterials has been improved and scaled up,

making affordable materi-

als available for test and

evaluation which did not

ex i st just 5 ye a rs ago.

C o mp u ter power has

i mp roved dra m a t i c a l ly

over the past 10 to 15

years, enabling atom-level

simulation of many new

nanomaterials before sig-

n i ficant re s o u rces are

i nve sted in pro d u c i n g

them. Researchers are now

able to theorize, simulate,

manufacture, test, and evaluate new materials in much shorter times, further

expanding the field and reducing was ted effort. While many of the ideas sur-

rounding engineering at the atomic and molecular level are still in a highly

speculative stage, experts agree that there seem to be no physical laws bar-

ring fur ther advances.

Many of the advancements in nanotechnology were made possible by

concurrent advancements in analysis tools capable of resolving atomic, molec-

ular and crystallographic structures. Just in the last 20 years, various forms of

scanning probe microscopy (SPM) and transmission electron microscopy

(TEM) have become available to researchers on a reliable basis. These tools,

described more fully in Part Two, have provided views into the inner workings

of atomic bonding, molecular assembly, and the structure of materials at the

smallest scales in history. It is these tools that have taken atomic manipulation

out of the chemistry beaker and into the realm of engineering.

Part Two: The How and Why of Nanotechnology

A Sense of Scale

To g rasp what nanotechnology encompasses, it is critical to have a mental

concept of what “nano” really means. Roger Whatmore of Cranfield University

in the UK provides an excellent example in which he proposes that one imag-

ine a human hair is roughly the size of a large tree trunk, one meter across.

Now a typical bacterium (1 micrometer) would be about as big as a cater-

pillar on that tree and a typical virus (which is about 100 nanometers long)

would be the size of an ant. And that is where nanotechnology is just g etting

started. Figure 2 presents an overview of items from the macro to the

nanoscale. One can see that multiple technical disciplines of chemistry,

physics, engineering, biology and others converge as we approach the lower

reaches of size.

Analysis and Tools

In order to study materials on the nanoscale, instruments capable of very fine

resolution must be utilized. Many of the microscopy tools used for character-

izing nanostructures can easily fit inside a shoebox. Advances in microscope

probe tip technologies have come about from the study of nanostructured

materials, creating even better tools for characterization. Figure 3 compares

some microscopy techniques and illustrates the incredible recent growth of

tools with micro- and then nanoscale resolution.

Scanning Probe Microscopy One of the first tools used for nanomaterials

investigation was the scanning tunneling microscope, or STM, developed by

Gerd Binnig and Heinrich Röhrer at IBM in 1981. (They would win a Nobel

Prize in 1986 for this work.) The STM formed the basis for what would later

become a family of scanning probe tools, or scanning probe microscopy

(SPM), the key features of which are depicted in Figure 4.

Scanning probe microscopes use an atomically “sharp” tip (typically pyram-

idal in shape and coming to a point that is literally one or two atoms across)

which is moved over a sample’s surface a few Angstroms from making contact.

The tip is then scanned across the surface in a regular pattern (much as one

would mow a lawn) and the electrical current, which traverses the gap

between the tip and the sample, is monitored. By keeping the height of the tip

constant and measuring the changing electrical current (more when close to

the sample, less when farther away), or by keeping the current constant and

measuring the deflection of the tip, a topographical map can be generated of

the sample’s surface. The device relies upon the small amount of electric cur-

rent which crosses the gap between the tip and the sample (“tunneling”).

The other most widely used member of the SPM family is the atomic force

microscope or AFM. Similar to the STM, it uses an atomically sharp tip, but

instead of carefully moving it just Angstroms from the sample, the AFM actual-

ly drags its tip along the sample’s surface. (This exaggerates the interaction at

this scale - the atomic forces actually prevent atoms from “dragging,” they

simply are pushed into very close proximity.) The tip is mounted on a cantilever

M a t e r i a lE A S E

Figure 1. Quantum Stadium

(Courtesy: IBM, Almaden

Research Center. All rights reserved)

Figure 2. Scale of Common Natural and Manmade Items

(Reprinted with permission of the Office of Basic Energy Sciences,

Office of Science, US Department of Energy)

which flexes as the tip moves up and down the surface topography. Its deflec-

tions can be tracked by bouncing a laser beam off the cantilever into a split

photodiode or by other techniques.

M a g n etic Fo rce Micro s c o py (MFM) is another te ch n i que of SPM, but

uses a magnetic probe to sense the magnetic field above the surface of a

m a te rial. The probe is sta n d a rd silicon- or silicon nitri d e - c o a ted with a th i n

m a g n etic film, such as cobalt, capable of mapping the magnetic domain of

a specimen with a resolution up to 20 nm. Two passes are re qu i red to dist i n-

guish the to p o gra p hy of the sample. The fi rst pass is in contact or semiconta c t

w i th the sample and the second pass is at a specific constant height above

the fi rst pass.

Electron microscopes have been instrumental in the study and characteri-

zation of materials since the invention of the first electron microscope in the

1930s. High resolution transmission electron microscopy can now provide

structural characterization at better than 0.2 nm spatial resolution. In its simplest

form, a TEM consists of an electron beam that is projected through a thin sam-

ple, generating a diffraction pattern onto a receiving device, such as a phos-

phorescent screen or CCD camera. At low resolution, amplitude contrast

images are used to map material structures in the 0.5 nm to 1 mm range.

Specimens must be < 10 nm thick to provide the highest resolution

Nanoscience

The promise of nanotechnology is based upon the ability to create nanostruc -

tured materials that will produce novel properties on the macroscale. That abil-

ity now exists, however in many cases the mechanisms behind these observed

properties are not yet clearly understood. Despite this incomplete understand-

ing, it is still possible to attribute, by inference, the novel qualities of these nano-

materials to the following changes in internal structure.

1. The increased total surface area of grains, as grain size is decreased, will

alter the physical properties of nanomaterials.

2. The increased volume of grain boundaries relative to unit structure, as grain

size is decreased, will alter the physical properties of nanomaterials.

3. Discrete electronic levels (quantum behavior) will alter the electrical and

optical properties of nanomaterials as grain sizes approach the molecular

scale. The discrete energies associated with electron orbits become more

evident as grain size nears the molecular scale (approximately < 5 nm),

creating non-linear property effects.

Mechanical Properties The most widely accepted hypothesis to explain the

mechanical properties in polycrystalline nanomaterials is that the strength and

hardness follow the Hall-Petch relationship, increasing with smaller grain size,

down to a critical grain size, dc ≅ 10 nm, where a decrease in strength and

hardness results thereafter, as depicted in Figure 5 [3].

The increase in strength is based upon the piling up of dislocations at g rain

boundaries; i.e. by increasing the total surface area of grains, the dislocation

density increases and in turn increases strength. However, pileups cannot

occur when the grain size is less than the dislocation spacing in the pileup [4];

and so the Hall-Petch relationship will no longer be valid on this scale. The

strength of nanoscale-thickness, multi-layered materials also follows the Hall-

Petch relationship, replacing grain size with layer thickness.

By decreasing grain size, the grain boundary volume relative to the unit vol-

ume will increase. At roughly 5 nm, 50% of the volume will be grain bound-

aries [5], which may then dominate the properties observed in the material.

The decrease in strength at this small end of the nanoscale may be attributed

to grain boundary sliding due to the high defect density allowing fast diffusion

of atoms and vacancies in the stress field [6]. Superplasticity [+] has been

observed in nanostructured me tals and ceramics at about 200°C lower than

microstructured materials. This creates improved formability of nanostructured

materials, which is especially impor tant for ceramics.

Thermal Properties The enhanced diffusivity observed in the grain boundary

structure of nanostructured materials is the mechanism thought responsible for

the changes in thermal properties. In metals, thermal conductivity and melting

point have been obser ved to decrease (such as a 27°C lower melting point

for gold) while thermal expansion coefficients have been observed to

increase. The decrease in thermal conductivity of nanostructured ceramic mate-

rials such as yttria stabilized zirconia may broaden their use as thermal barri-

er coatings.

Chemical Reactivity An increased chemical activity can be obtained by the

large number of atoms on the sur face of nanocrystallites providing active sites

for reactions. For instance, nonstoichiometric CeO2-x has a high chemical reac-

tivity resulting from an unusually high oxygen vacancy concentration [7].

Nanostructured CeO2-x has been demonstrated to offer catalytic activation for

SO2 reduction and CO oxidation at significantly lower temperatures as well

as enhanced poisoning resistance (resistance to the loss of catalytic reactivity).

A second class of reactive materials are nanostructured porous materials.

Conventional porous materials such as aluminosilicates and phosphates are

Figure 4: Graphical Depiction of the Key Features of an SPM

1800 1850 1900 1950 2000

Year

106

105

104

103

102

10

1

ROSS

AMICI

ABBE RUSKA

ASH

REASONYOUNG

BETZIG

BINNIG

DIETRICHBINNIG & ROHRER

Fine probe positioning and

deflection measurement

Course positioning structure

Stage

Measure electricalcurrent between

probe and sample

Probe tip

Atomic surface

Figure 3: History of Lateral Resolution in Microscopy

(Courtesy of J. Murda y, Naval Research Laboratory)

AMPTIACA D VA N C E D M A T E R I A L S A N D P R O C E S S E S T E C H N O L O G Y

MARTON

used for catalytic reactions and gas absorption applications, but they are lim-

ited due to their small pore sizes of less than 15 Å [8]. Nanostructured alumi-

nosilicates with optimized pore sizes in the range of 20–100 Å have been

developed which extend their use in these markets. Similar research is being

applied to the use of transition metal oxides in petrochemical production, pol-

lution control, and pharmaceutical and fine chemical synthesis. Extending the

pore sizes beyond the 20–100 Å range could provide benefits for enzyme

catalysis, bioseparation and biosensing.

Optical Properties The size of dispersion materials in a composite can alter

the wavelength of light that is absorbed by the particulates. The quantum effect

is responsible here where the discrete electron energies in the particulates

determine the wavelength of light absorbed. Altering the size of a particle can

change the associated energy and wavelength of light absorbed. Cadmium

selenide is one material extensively studied where crystallites of ~ 1.5 nm will

appear yellow, 4 nm will appear red, and larger particles will appear black

[9]. This discovery is now being applied to sunscreen lotions using zinc oxide

and titania to filter ultraviolet radiation.

Electrical Properties Quantum ef fects in nanomaterials have been shown to

produce a non-linear dependence of electrical conductivity on electric field

and can produce electron tunneling characteristics. Quantum mechanical inter-

ference between separate paths an electron may take through a material can

strongly enhance or suppress electrical conductivity. Most of the research to

date in nanostructured electronic devices comes from fabrication via lithogra-

phy, but inroads are being made into molecular-level structures fabricated from

chemical reactions or self assembly. For ins tance, quantum dots are pyramid-

or faceted dome-shaped clus ters of atoms, typically between a few nanome-

ters and hundreds of nanometers in diameter. They self-assemble from the dep-

osition of a large lattice constant material onto a substrate with a small lattice

constant, whereby the compressive strain in the deposited material relieves

itself by causing the material to spontaneously coalesce or clump into islands

on the substrate. They may be utilized as three-dimensional potential wells if

they are overgrown by a material with a larger bandgap and have applica-

tion potential in nanoelectronics. (See the article in this issue by Dr. Amirtharaj

et al for fur ther information and application of quantum dots.)

CNTs (the tubes formed from rolled up sheets made of carbon atoms joined

in hexagonal arrays) have shown some interesting electrical properties as well.

The way in which the two ends of the sheet wrap and meet can modify the lon-

gitudinal conductivity of the tube. In fact, they can be made to conduct freely

like metals or behave like semiconductors. And further advancements have

shown that various-sized tubes can be nested inside one another for structural

or electrical performance modifications.

Magnetic Properties Magnetic nanoparticles exhibit unusual behaviors result-

ing from the size effects and charge transfer characteristics. Changes seen in

these materials include Giant Magneto-Resistance (GMR) in some layered

composite materials and g ranular solids, spin valves, and spin injection in fer-

romagnet/insulator/ferromagnet sandwich materials.

Part Three: Processing of Nanomaterials

Bulk Methods

A number of processes are used for producing nanomaterials in bulk powders,

coatings, thin films, laminates, and composites. However, there are two funda-

mental approaches to fabricating nanomaterials. The “bottom-up” approach

represents the concept of constructing a nanomaterial from basic building

blocks, such as atoms or molecules. This approach illustrates the possibility of

creating exact materials – materials that are designed to have exactly the

properties desired. The second approach, the “top-down” method, involves

restructuring a bulk material in order to create a nanostructure.

INERT GAS CONDENSATION C o n s i d e red a “bot tom-up” method, inert ga s

condensation (IGC) was the fi rst method used to inte n t i o n a l ly produce a nanos-

t ru c t u red mate rial, and has become widespread. It can be used to pro d u c e

n a n o st ru c t u red metals, alloys, inte rm etallics, ceramic oxides, and comp o s i te s .

The process begins by energizing a source mate rial, which ge n e ra tes a va p o r

of atoms. This eva p o ration process can be perfo rmed th rough electron beam

heating, laser ablation, sputte ring, or plasma methods, but is most often done

th rough Joule-heating. Due to condensation, the va p o rized atoms aggl o m e ra te

and fo rm ve ry small cluste rs when th ey are introduced to the inert gas. These

c l u ste rs are carried by a conve c-

tion current, induced by th e r-

m o p h o resis, to a tube cooled by

l i quid nitro gen where th ey accu-

m u l a te .

Adjusting certain parameters,

such as the gas used (i.e. He, Ar,

Kr, Xe), the gas pressure, the pre-

cursor evaporation rate, and the

residence time, allows some con-

trol over the particle size and dis-

t ribution. The smallest part i c l e

sizes produced using this method

are about 5–25 nm using a pre-

cursor with a low evaporation rate

Figure 5: Strength/Hardness of Nanostructured Materials [3]

H, σy ∝ d -1/2

amorphous

Grain Size, ddc

Hall-Petch Equations

σy = σo + Kd -1/2 H = Ho + Kd -1/2

σy: yield strength H : hardness

σo: lattice yield stress Ho: single crystal

(minimum stress hardness

required to move

a dislocation)

K: constant

d: average grain size


AMPTIACAD VA N C E D M AT E R I A L S A N D P R O C E S S E S T E C H N O L O G Y

and a light inert gas at low pressure [10]. Since the process takes place in

ultra-high vacuum systems (UHV), there is very little contamination of the mate-

rial, and therefore, it remains highly pure.

MECHANICAL ALLOYING Another process commonly used to produce

nanos tructured materials is mechanical alloying, which is an example of a

“top-down” method. There are two basic versions of this process. The first, more

specifically called mechanical attrition, is a process where high-energy ball

milling is used to grind and refine the structure to produce highly mixed, ultra-

fine powders. Grain sizes of the material are partially dependent on time,

where extended milling times result in more uniform grain sizes. The second

version, reaction milling, involves in-situ solid-s tate chemical reactions between

precursor materials while they are mixed and milled.

Producing large quantities of nanostructured material is one advantage of

the mechanical alloying process. Moreover, various types of nanomaterials,

such as metals, ceramics, intermetallics and composites, can be produced

using these straightforward processes. However, contamination from the milling

hardware and environment, non-uniform particle sizes due to short milling

times, and non-homogeneous chemical composition due to incomplete milling

reactions are problems for mechanical alloying processes. One way to lessen

the effects of environmental contamination is to perform the process in the pres-

ence of liquid nitrogen, which is also refer red to as cryogenic milling.

SEVERE PLASTIC DEFORMATION Severe plastic deformation (SPD) is used to

fabricate nanocrystalline metals and intermetallics. There are several different

types of SPD processes, but the three most common processes are equal-chan-

nel-angular extrusion (ECAE), torsional straining and accumulative roll-bonding

(ARB) [10]. Nanosized grains are created when the material is subjected to a

very large deformation causing a modification of its structure by fragmenting

the existing phases. Recrystallization of these broken phases results in structures

with significantly reduced sizes. The materials formed by SPD generally have

average grain sizes of about 100 nm, although grain sizes down to 20 nm

have been obtained for metals [11]. The grain size controlling parameters in

this process include temperature, strain, strain rate, and applied pressure.

Severe plastic deformation is capable of forming nanostructured materials with

little contamination and little or no porosity. A further advantage is that SPD is

a scalable process, meaning that it could be used for industrial applications.

However, since there is significant straining of the material, high residual stress-

es may be present in the end product.

SOL-GEL The sol-gel method is a solution phase processing technique that

is used more of ten to fabricate nanostructured materials than any other liquid

phase process. This is a process capable of producing nanostructured ceram-

ics and nanocomposites. There are essentially two phases present during this

technique, where one is a homogeneous solution phase and the other is an

elastic, gel-like, solid phase. The homogeneous solution is dried, thereby trans-

forming it into a gel, while maintaining a constant volume. Subsequent drying

causes a phase transformation of the gel along with a corresponding reduc-

tion in volume, ultimately resulting in the desired phase. The key to obtaining

a nanostructured material using this process is to control the processing param-

eters. One advantage to solution phase processing is the excellent control it

provides over the chemical composition, which leads to a more homogeneous

composition [10]. A disadvantage to this process is that the starting materials

can be expensive.

Coating and Laminate Methods

Nanostructure coatings have been primarily deposited via thermal spraying.

Processes used to fabricate laminates include: Radio Frequency sputtering, DC

magnetron sputtering, chemical vapor deposition, electroplating, and physical

vapor deposition methods including electron beam, cathodic arc, and jet

vapor deposition [12].

THERMAL SPRAYING Thermal spraying is a process capable of producing

nanostructured materials and nanostructured coatings. Control over the com-

position and structure of the final material is very important when producing

nanostructured materials, and for this reason powders are typically used as the

starting materials. The powders are carried by a gas (i.e. air, N2) and heated

such that they are at least partially melted. Deposition onto a substrate results

in the deformation and solidification of the particles.

ELECTRODEPOSITION Electrodeposition has traditionally been used as a

coating process, although recently it has also been used to form bulk nanocrys-

talline materials. This process is capable of producing a nanocrystalline mate-

rial with grain sizes down to 5 nm. The factors affecting the resulting grain sizes

include pH, temperature, current density, and type of current among others

[10]. The process can be carried out at room temperature and the time of fab-

rication is of course dependent on the size of the material to be produced. For

example, as one would expect, larger materials take longer, but compared to

other deposition processes, electrodeposition is capable of rapid deposition.

Moreover, electrodeposition is a less expensive process that can be used for

large-scale production [10].

JET VAPOR DEPOSITION Jet vapor deposition is a relatively cost-effective

technique capable of rapidly depositing multilayered films with alternating

material layers to produce a nanostructured composite [13]. Nanostructured

multilayered films consist of layers with nanoscale thickness. Sonic or nearly

sonic gas jets, typically helium, carry an atomized material and deposit it as a

film on a substrate. Uniform deposition occurs with a rotating or oscillating sub-

strate which is mounted on a carousel. Alloys and multilayered composites are

formed by employing two or more jets simultaneously or in sequence, respec-

tively. An optimum deposition temperature needs to be maintained to ensure

minimal interdiffusion between the layers and a continuous nature of the layers

without impurities forming in between.

SPUTTERING Sputtering is a well-known and widely used technique for pro-

ducing thin films. It is a vacuum method capable of producing nanostructured

materials. In this technique, plasma, which is generated by energizing a low-

pressure gas such as argon, strikes a target transferring its momentum which

causes the target material to be ejected and subsequently deposited onto a

substrate. The main disadvantage to the sputtering method is that it is difficult

to control. DC-magnetron sputtering and RF-diode sputtering have been used

to prepare nanolaminate materials. These methods are capable of depositing

layers of a material with a thickness below 100 nm.

CHEMICAL VAPOR DEPOSITION The process of chemical vapor deposition

(CVD) involves a gas-phase chemical reaction which forms a solid material on

a substrate. Nanostructured ceramics and composites are the most common

types of nanomaterials produced by CVD (Carbon nanotubes are also fabri-

cated using CVD). Vapors of the precursor materials are transported by a car-

rier gas to a heated substrate where they are deposited and subsequently

react with the substrate to form a solid material. Although it is a relatively slow

process, chemical vapor deposition offers good control over the chemical


composition and is capable of deposition over a large area. There are sever-

al variations of the CVD process, such as plasma assisted CVD and laser

assisted CVD.

CHEMICAL VAPOR INFILTRATION Chemical vapor infiltration (CVI) is a tech-

nique that is used to form nanostructured fiber-reinforced composite materials.

A very similar process to CVD, CVI uses a transporting gas to carry vaporous

precursors to a porous fiber preform, while diffusion takes the precursors inside

where they react and form the nanostructured matrix. An important issue with

CVI is ensuring that the outer parts of the fiber preform are not sealed off prior

to the interior being fully deposited. To prevent premature surface densification,

the chemical vapor infiltration process is slowed by adjusting the process

p a ra m ete rs, such as te mp e ra t u re, pre s s u re, and re a c tant concentra t i o n .

Another factor to CVI being a slow process is that it is limited by the diffusion

mechanism of the gaseous precursors. The advantage to CVI is that it is capa-

ble of producing a fiber-reinforced composite without damaging the fibers,

which is often a problem in other composite processes.

CHEMICAL VAPOR CONDENSATION Another chemical vapor process used to

produce nanomaterials is chemical vapor condensation (CVC). A precursor

vapor carried by a gas stream is sent through a heated (hot-wall) reactor tube,

where the pyrolysis of the precursor occurs to form nanoparticles. The

nanoparticles are then collected on a liquid-nitrogen cooled tube. The CVC

process is a low-pressure technique that requires a low concentration of the

precursor in the carrier gas stream. Process parameters such as temperature,

pressure, and residence time can be adjusted to determine the size of the

nanoparticles. A variation of this process is the combustion flame version which

replaces the hot-wall reactor with a combustion flame reactor. The combustion

flame reactor supplies a significantly higher temperature which increases the

reaction rate.

For further coverage of the subject matter, the interested reader can find

more information on nanostructured materials in “Nanomaterials: Synthesis,

Properties and Applications” [14].

References and Endnotes

[1] R. Birri n ge r, “St ru c t u re of Na n o st ru c t u red Mate ri a l s ,” Na n o p h a s e

Materials: Synthesis – Properties – Applications, Proceedings of the NATO

Advanced Study Institute on Nanophase Materials: Synthesis – Properties –

Applications, Kluwer Academic Publishers, 1994, pp. 157-180.

[2] “The Development of Achievable Machining Accuracy”, from Current

Status in, and Future Trends of, Ultraprecision Machining and Ultrafine

Materials Processing, by Norio Taniguchi, Tokyo Science University, Annals of

the CIRP Vol. 32/2/1983 page 573.

[3] T.G. Nieh and J. Wadsworth, Scripta Met., Vol. 25, 1991, pp. 955-958.

[4] V.G. Gryazanov, V.A. Solo’ev and L.I. Trusov, Scripta Met. Mater., Vol 24,

1990, pp. 1529-1534

[5] C. Suryanarayana and F.H. Froes, “The Structure and Mechanical

Properties of Metallic Nanocrystals”, Metall. Trans,, Vol 23A, 1992, pp.

1071-1081.

[6] S. Veprek, “Ultra Hard Nanocomposite Coatings with Hardness of 80 to

105 GPa”, Int. Conf. on Trends and Applications of Thin Films, TAFT 2000,

Nancy, France, March 2000.

[7] A.S. Tschöpe and J.Y. Ying, Nanostr. Mater., Vol.4, 1994, p. 617.

[8] J.Y. Ying, “Nanostructure Processing of Advanced Catalytic Materials”,

WTEC Study on Na n o p a rticles, Na n o st ru c t u red Mate rials, and

Nanodevices. Part 1. Proceedings of the 1997 US Review Workshop,

A rl i n g ton, VA, May 19 97, National Te chnical Info rmation Service, US

Department of Commerce.

[9] R.W. Whatmore, “Nanotechnology”, Ingenia, February 2001.

[10] L.L. Shaw, “Processing Nanostructured Materials: An Overview”, JOM,

Vol. 52, No. 12, The Minerals, Metals & Materials Society, December 2000,

pp 41-45.

[11] R.Z. Valiev, I.V. Alexandrov, R.K. Islamgaliev, “Processing and Properties of

Nanostructured Materials Prepared by Severe Plastic Deformation”, NATO

Ad vanced Study Inst i t u te on Na n o st ru c t u red Mate rials: Science &

Technology, Kluwer Academic Publishers, July 1999, pp. 121-142.

[12] M. Gell, “Applying Nanostructured Materials to Future Gas Turbine

Engines”, JOM, Vol. 46, No. 10, The Minerals, Metals & Materials Society,

October 1994, pp. 30-34.

[13] H.N.G. Wadley, L.M. Hsiung, and R.L. Lankey, “Artificially Layered

Na n o c o mp o s i tes Fa b ri c a ted by Jet Vapor Deposition,” Comp o s i te s

Engineering, Elsevier Science Ltd, Vol. 5, No. 7, 1995, pp. 935-950.

[14] A.S. Edelstein, R.C. Cammarata, Nanomaterials: Synthesis, Properties

and Appications, Inst. of Physics, October 1998.

* First published in the February 1960 issue of Caltech’s Engineering and

Science, which owns the copyright. It has been made available on the web at

http://www.zyvex.com/nanotech/feynman.html with their kind permission.

† Norio Taniguchi shows the trend in ultraprecision machining from 1900 and

projects that it will reach “Atomic lattice distance” (0.3 nanometers) around

2017.

+ Superplasticity is the ability of a material to exhibit large ductility, usually >

200%.


Introduction

Modifying material surfaces to enhance wear and corrosionresistance is a common practice for both military and commer-cial applications. Electrodeposited hard chrome is one of themost widely used protective coatings. Ceramic coatings, bothsingle phase and composite types, are also common and theyare frequently applied using plasma spray. In this process, thecoating material (usually in the form of a powder) is injectedinto a plasma stream where it is heated and accelerated towardthe substrate surface. After impacting the surface the ceramicrapidly cools thus forming a coating layer.

Both hard chrome and ceramic coat-ings have serious deficiencies that canlimit their use. Chrome electroplatinguses closely regulated hazardous materi-als. Compliance with the various environ-mental safety regulations has made hardc h rome increasingly expensive to use.Pl a s m a - s p r a yed ceramic coatings aresomewhat less expensive than chrome(when clean up costs are included), butare generally brittle and have limited suc-cess adhering to substrates, which is also aproblem for hard chrome. The need forbetter coating materials has been recog-nized and considerable effort has recentlygone into finding replacements.

Over the last five years, a consortium ofcompanies, universities and Navy person-nel have been developing a new genera-tion of wear resistant “nanostructured”ceramic coatings. The consortium is leadby Inframat, Inc. and the University ofConnecticut, and team members includethe A&A Company, Rutgers University,St e vens Institute of Te c h n o l o g y, theNaval Surface Warfare Center (CarderockDivision) and Puget Sound Na va lShipyard. It is funded by the Office ofNaval Research and its objective has beento achieve mechanical and wear proper-ties unobtainable from more convention-al materials (i.e. materials with structuralaspects at the micron scale or larger).

Nanostructured materials are characterized by an ultra-finem i c ro s t ru c t u re with some physical aspect less than 100nanometers in size. This feature can be grain size, particle orfiber diameter, or layer thickness (Figure 1). There are two rea-sons why reducing the scale of a material’s microstructure cansignificantly alter its properties. First, as grain size gets smaller,the proportion of atoms at grain boundaries or on surfacesincreases rapidly. In a polycrystalline material with a grain sizeof 10nm, as many as 50% of its atoms are at grain boundaries,resulting in a material with properties far different than nor-

Figure 1. Nanostructured Materials are Characterized by the Inclusion

of One or More Types of Features with Dimension Below 100 Nanometers

Dr. Lawrence T. KabacoffMaterials Science and Technology Division

Office of Naval Research

Particle Diameter Layer Thickness

Fiber Diameter Grain Size

mally seen. The other reason is related to the fact that manyphysical phenomena (such as dislocation generation, ferro-magnetism, or quantum confinement effects) are governedby a characteristic length. As the physical scale of the mate-rial falls below this length, properties change radically. Untilrecently, these changes in deformation behavior and modesof failure have not been well understood due to the inabili-ty to consistently fabricate high quality materials. This situ-ation is changing rapidly, with considerable progress nowbeing made in the fabrication of nanomaterials, as well asand the understanding of the interrelations betwe e nnanoscale processing, structure, and macroscale properties.

Fabrication of Nanoceramic Coatings

The strategy employed to develop nanostructured coatingsconcentrated on compositions similar to currently availableconventional coatings and to use existing deposition equip-ment to fabricate them. As only the microstructure of the coat-ings was changed, it greatly simplified the process of imple-menting the new technology in both military and commercialapplications. One of the coatings under development, a plasmasprayed nanoceramic composite with a composition of Al2O3 -13 TiO2, has exhibited wear resistance, bond strength, andtoughness unprecedented in a ceramic, and is now in useaboard Navy surface ships and submarines, reducing mainte-nance costs due to wear and corrosion.

Plasma spray, the process used to fabricate ceramic coatings,is very simple in concept, but very complex in practice. Aninert gas is passed through a region of electrical discharge,where it is heated to very high temperature (typically 10,000 to20,000 K). The rapidly expanding plasma is forced out througha nozzle at velocities between 1,200 and 1,500 m/sec anddirected toward a substrate. Particles are injected into the plas-ma, where they are heated and accelerated. Although the plas-ma and particle temperatures are high, substrate surface heatingis minimal. A schematic of a typical plasma spray gun is shownin Figure 2. The complexity arises from the large number ofparameters that must be selected and which can affect thestructure and properties of the coating. The temperature andvelocity of the plasma depend on the power applied to the gunand the type and flow rate of the gas used. Usually, two gassesare used, an inert gas such as helium or argon, and a secondarygas, such as hydrogen. Other factors include the morphology ofthe powder particles, distance from the gun to the substrate,position and orientation of the powder injection ports, and sur-face preparation of the substrate. Taken all together, these


Figure 2. Schematic Diagram

of Typical Plasma Torch

(Courtesy of R.W. Rigney)

Figure 3. Indentation Crack Growth

Resistance vs. Critical Plasma Spray

Parameter (proportional to plasma

temperature) [3]

Figure 4. Effect of Nanoparticle

Agglomerate Melting on

Coating Microstructure [3]

Tungsten

Cathode

Nozzle

Copper

Anode

Plasma

Spray Deposit

Spray Stream

Prepared

Substrate

Electric

Arc

Powder and

carrier gas

Electrical (+)

Connection

and Water In

Arc gas

Electrical (-)

Connection

and Water Out

290 320 350 380 410 440

CPSP

(A Parameter which is proportional to Temperature

CPSP=Applied Power/Primary Gas Flow Rate)

14

12

10

8

6

4

2

0

Not Melted

Partially Melted

All Melted

Starting

Microstructure

Coating

Microstructure α-Al2O3 (0.5 ~ 2.0 µm)

γ-Al2O3 with dissolved Ti4+ (20-100 nm)

Oxide solution of TiO2 and/or oxide additives (20-100 nm)

2-1/2 in. - 6 in.

(64 to 152 mm)

nanostructured

conventional

parameters determine the thermal history of the injected parti-cles, velocity of impact, and flow and solidification characteris-tics after impact, thus dictating the resultant microstructure.

As compared to traditional plasma spray processes, plasmaspraying nanostructured materials introduces a number ofcomplications. The first is that nanoparticles cannot be sprayedby particle injection into the plasma. Very small particles lackthe momentum necessary to penetrate into the plasma, or toimpact the surface while the plasma sweeps to the side near thesubstrate. To be sprayed, the particles must be formed intoagglomerates approximately 30-100 microns in diameter. Foran Al2O3 - TiO2 nanocomposite, this is usually accomplishedby dispersing alumina and titania nanoparticles in a fluid witha binder and spray drying [1]. If necessary, the agglomerates arepartially sintered to improve structural integrity.

The next problem is forming a nanostructured coating onthe substrate. This is not trivial since the agglomerates are great-ly heated (promoting rapid grain growth) and are at least par-tially melted. There are three mechanisms for creating or retain-ing a nanoscale microstructure: avoiding melting or grain

growth of the feedstock (very difficult), inclusion of nanoscaleparticles with very high melting temperature that remain solidwhile the rest of the material melts, or formation of a nanos-tructure during solidification of the sprayed material uponimpact. The last mechanism occurs in composites consisting oftwo or more immiscible phases (as is the case for Al2O3 andTiO2) and results from solid state decomposition of a single,metastable phase formed by rapid solidification during impact.The metastable phase formed by Al2O3 and TiO2 is a highlydefected Spinel [2].

The microstructure and properties of nanostructured Al2O3

- 13TiO2 coatings depend strongly on the temperature of theplasma [3]. This is in sharp contrast to the conventional coat-ing, as illustrated in Figure 3, which shows a plot of indenta-tion crack resistance as a function of a parameter (appliedpower divided by primary gas flow rate) which is proportionalto the plasma temperature. Data is shown for both the nanos-tructured and conventional coatings. One can see that thecrack growth resistance for the conventional material is virtual-ly independent of plasma temperature while the nanostruc-tured coating exhibits a strong dependence.

The reason can be found by following the thermal history ofthe injected particles. In the plasma spraying of conventionalAl2O3 - 13TiO2, the feedstock consists of large fused andcrushed particles that are fully melted prior to impact. Fullmelting is achieved over a wide range of temperature because ofthe high thermal conductivity of fused and crushed particles(relative to nanoparticle agglomerates). The coatings typicallyhave grain sizes greater than one micron. When the nanoparti-cle agglomerates are sprayed at relatively high temperature(under conditions that result in full melting), the “nano” struc-ture consists entirely of grains formed from the decompositionof the metastable Spinel phase.

The difference between the micro-grained material formedf rom fused and crushed feedstock and the nanostru c t u re formedf rom the agglomerates arises from the degree of homogeneity inthe melted particles. Because of the short residence time, the liq-


Figure 5. Cup Test Performed on Conventional (Left)

and Nanostructured (Right) Al2O3 -13TiO2 Coatings

(Courtesy of R.W. Rigney)

Figure 6. Cross-

Sectional Views of

Crack Propagating

in Conventional

Al2O3-13TiO2

Coating [3]

50µm15µm

40

uid formed from melting the fused and crushed particles willcontain alumina and titania rich regions. When this liquid solid-ifies rapidly, the grains formed will be larger than would beobtained from a homogeneous liquid. When plasma spray iscarried out at a re l a t i vely low temperature, little melting occursand the stru c t u re consists of incompletely sintered particles thath a ve undergone some degree of grain growth. The middle tem-p e r a t u re range gives partial melting of the agglomerates andresults in regions of micron sized grains (from unmelted part i-cles that have experienced grain growth) surrounded bynanoscale grains from the fully melted material and is illustrat-ed in Fi g u re 4. It is this “d u p l e x” stru c t u re that gives the bestp ro p e rties, not a homogeneous nanoscale stru c t u re.

Properties of Nanostructured Al2O3 - 13TiO2 Coatings

The obvious parameter by which to judge a “wear resistant coat-i n g” is wear rate. Wear can be termed as either sliding or abra-s i ve. Both are measured by running a “we a r i n g” medium ove rthe surface and measuring weight loss. For many coatings, andp a rticularly for brittle materials such as ceramics, this parametercan be misleading. The wear resistance of coatings in use todayis outstanding, with wear rates orders of magnitude less than theuncoated surface. Brittle coatings usually do not fail by “we a r i n go u t”, but rather suffer from cracking, delamination and spalla-tion. At least as important as wear resistance is bond stre n g t h(adhesion to the substrate) and toughness (the ability to with-stand an impact or applied strain). It is in these pro p e rties thatthe nanoceramic coatings excel to a re m a rkable degre e .

The bond strength of the nanostructured coatings, as meas-ured by tensile pull tests on coatings applied without a bondcoat, is about double that of a conventional coating. Thetoughness of the nanostructured Al2O3 - 13TiO2 coatings isextraordinary, as is dramatically illustrated in Figure 5. Thisshows a “cup test” in which a coated coupon is forced downonto a one-inch diameter steel ball (coated side away from theball), causing it to deform. The deformation is greatest in thecenter decreasing to zero at the edges. The conventional coat-

ing shows the typical cracking and spalling observed in aceramic. The nanostructured coating actually deforms alongwith the substrate and no macroscopic cracking is observed. Ablow from a hammer severe enough to deform a steel substratewould not be sufficient to cause failure in the coating. Thistoughness translates into greater wear resistance, which is twoto four times greater than that of the conventional coating [4].

Another important benefit of enhanced coating toughness isimproved grindability as almost all ceramic coatings must beground and polished after spray deposition. Nanostructuredceramic coatings can be ground and polished in about half thetime required for conventional ceramic coatings. Since grind-ing and polishing operations represent about forty percent ofthe total cost of the coating (compared to about five percent forthe cost of the feedstock powder), nanostructured coatings areactually less expensive to apply.

The performance of the conventional and nanostructuredcoatings can be better understood by examining how crackspropagate in these materials. In the conventional coating,cracks follow the “splat boundaries” which mark the bordersbetween the materials deposited from each droplet (Figure 6).The arrows in this figure point towards a splat boundary. Acrack can be seen following another splat boundary [3]. In thenanoceramic composite, the cracks do not follow the splatboundaries, but instead propagate through the nanostructuredmaterial that formed during solidification until they encountera region of larger grains formed from partially melted feed-stock. Here, the cracks are either deflected or stop inside the“coarse grained” region. This can be clearly seen in Figure 7,which illustrates that cracks terminate inside regions (seearrows) of large grains formed from unmelted feedstock [3].Any strain imposed upon a nanostructured coating is accom-modated by the creation of microcracks in the normal way, butthese cracks are blunted before they can propagate very far orlink up with other cracks. The result is a ceramic material thatcan deform to a degree much greater than in a more conven-tional, brittle ceramic.

Figure 7. Cross-

Sectional Views of

Crack Propagating in

Nanostructured Al2O3 -

13TiO2 Coating With

“Duplex Structure” [3]


10µm 50µm

Applications

The applications for nanostructured Al2O3 - 13TiO2 coatingscan be grouped according to the properties to be exploited. Thesimplest applications are those in which the nanostructuredcoatings replace existing conventional ceramic coatings. In suchcases, the advantage is greater longevity and reliability. The sec-ond case is as a replacement for hard chrome coatings. Here,the advantages are lower cost, elimination of toxic hazardousmaterials and in some cases, improved performance. Theimproved performance comes from the non-metallic nature ofceramics. Metallic surfaces in contact with seawater for longperiods of time experience buildup of calcareous deposits.Actuator rods with such buildups can severely damage seal areaswhen the rods are retracted. Electrically non-conducting coat-ings experience no such buildup. Also ceramics do not promotegalvanic corrosion, which occurs when two dissimilar metalsare in contact with an electrical conductor such as seawater. Inspite of these advantages, conventional ceramic coatings fre-quently cannot be used over metallic coatings because they lacksufficient bond strength and toughness. Nanostructured ceram-ics overcome this limitation. In fact, nanostructured Al2O3 -13TiO2 coatings are now being used to coat titanium compo-nents on seawater-exposed portions of submarines specificallyto eliminate galvanic corrosion on nearby steel structures. Themost unique applications are in circumstances where no coat-ings could previously be employed. For example, many shaftsused aboard ships, such as propulsion shafts, undergo sufficienttorsional strain to cause failure in a conventional coating.Nanostructured Al2O3 - 13TiO2 coatings exhibit sufficientstrain tolerance to be a viable candidate for coating of severewear areas on certain types of shafts.

The qualification of a new material for military applicationscan be a fairly invo l ved process. This process has been gre a t l ysimplified for nanostru c t u red Al2O3 - 13Ti O2 coatings becausethe coatings are similar in composition to existing approved coat-

ings and are applied using the same equipment and pro c e d u re s .They have now been approved under MIL STD 1687A, whichg overns use of thermal spray coatings on shipboard machinery.The number of applications in use aboard surface ships and sub-marines is rapidly expanding. T h e re are literally thousands ofpotential applications and the impact on the cost of maintainingships, aircraft, and ground vehicles can be ve ry large. This can bedemonstrated by examining a typical example.

The component illustrated in Figure 8 is a reduction gear setfrom an 80-ton air conditioning unit used on surface ships.The coated areas of these gears are indicated by the arrows.Currently gears are replaced on average at six year intervals.Eventually, abrasives form on the shaft, excess heat is generatedand it welds to the aluminum sleeve in which it rotates, thuscausing it to seize up. With application of the new coatings,these gears can be repaired instead of replaced. The damagedsurface is ground down and replaced with a nanostructuredAl2O3 -13TiO2. The savings fleet-wide from this one applica-tion is about $500,000 per year, or about $13,000,000 over theprojected thirty-year life of these ships. When one considers thelarge number of other applications, such as pumps, valves, elec-tric motors, diesel engines, bearings, journals and actuators, themagnitude of the cost savings becomes apparent. Moreover,most of these components are not military-specific. Coatingsare now being applied to commercial hardware as well.

While nanostructured ceramic particles and thin films havebeen available for some time, nanostructured Al2O3 -13TiO2

represents the first bulk ceramic material to be commercialized.Other nanostructured coating materials are under developmentand are expected to become available in the near future. Theseinclude cemented carbides, such as Tungsten Carbide-Cobalt(WC-Co) and other ceramics, such as chrome oxide and yttria-stabilized zirconia. All of these coatings are expected to find awide range of application and should greatly benefit both mil-itary and commercial operating and maintenance costs.


Figure 8. High Speed

Reduction Gear Set

From 80 Ton Air

Conditioning Unit.

Coated Areas are

Marked with an

Arrow (Courtesy of

R.W. Rigney)


References

[1] L. Shaw, D. Goberman, R. Ren et al., Surf. Coat. Technol.Vol. 130 (2000) 1.[2] B.H. Kear, Z. Kalman, R.K. Sadangi, G. Skandan, J.Colaizzi, W.E. Mayo, J. Thermal Spray Technol. Vol. 9 (2000)483.

[3] M. Gell, E.H. Jordan, Y.H. Sohn, D. Goberman, L. Shaw,T. D. Xiao, Surf. Coat. Technol., In Press.[4] K. Jia, T.E. Fischer, Wear Vol. 200 (1997) 310.

Dr. Lawrence T. Kabacoff is a Program Officer in the Materials Science and Technology Division, at the Office of Naval Research,

where he directs programs in synthesis and processing of nanostructured materials, electronic packaging, and IR transparent mate-

rials. He obtained a Ph.D. in Solid State Physics from the University of Connecticut in 1978. He then served as a Post Doctoral

Research Fellow at Northeastern University until 1979, where he conducted research into the origins of glass formation and sta-

bility in me tallic glasses. Subsequently, he joined the Naval Surface Warfare Center, White Oak, as a research Physicist in the

Materials Division, where he conducted research on magnetoelastic metallic glasses and shape memory alloys. Since 1991, he

has served as a program of ficer in the Materials Division of the Office of Naval Research. He has managed programs in the

areas of acoustical damping in structural materials, fabrication of optical quality diamond, multi-chip module packaging, improve-

ment of high temperature mechanical properties of sapphire (for IR domes), and synthesis and processing of nanostructured alloys

and ceramics.

Introduction

Energetic materials are a major component of weapons systemsused by all branches of the US military. Their primary use is inexplosives, as well as in gun and missile propulsion. Over thelast century new chemicals have been discovered or designedfor rapid release of energy in either relatively simple composi-tions, like that used in certain warheads, or in more complexformulations like the advanced composites used as propellants.Currently, a class of energetic materials known as nitramines areactually used for both explosives and propulsion applications invarious weapons systems. Some major considerations for suc-cessful weaponization of energetic materials include perform-ance (e.g. energy density, rate of energy release), long-term stor-age stability, and sensitivity to unwanted initiation.

As demands on munitions increase with regards to improvedperformance (i.e. increased lethality and survivability as well asthe development of emerging high precision weapons con-cepts), the challenge on the R&D community is ever increas-ing. Additional drivers and concerns for the US military comefrom the continuing development of new munitions (includingnew types of energetics) by foreign nations. Munitions devel-opment is also fundamentally impacted by the approachinglimit of the amount of improvement that is possible for the tra-ditional and now rather mature C, H, N,and O energetic chemistries.

In recent years researchers have foundthat energetic materials/ingredients that areproduced on the nanoscale have the prom-ise of increased performance in a variety ofways including sensitivity, stability, energyrelease, and mechanical pro p e rties. Assuch, they represent a completely new fron-tier for energetic material research anddevelopment with the potential for majorpayoffs in weapons systems. Very simply,nanoenergetics can store higher amounts ofenergy than conventional energetic materi-als and one can use them in unprecedentedways to tailor the release of this energy so asto maximize the lethality of the weapons.The field of nanoenergetics R&D is quiteyoung, but is already undergoing rapidgrowth. The goal of this article is to givethe reader a sense for the physical andchemical characteristics and properties thatmake these materials so promising. This

will be accomplished through a discussion of a few selectedexamples of current research.

Background

The 221st National Meeting of the American Chemical Societyheld during April 2001 in San Diego featured a symposium onDefense Applications of Nanomaterials. One of the 4 sessions wastitled Nanoenergetics. This session featured speakers from gov-ernment labs (DOD and DOE) and academia (for furtherinformation about this symposium, please contact the author).This session provided a good representation of the breadth ofwork ongoing in this field, which is roughly 10 years old. Anumber of topics were covered, including a few that will be dis-cussed in detail below, namely Metastable In t e r m o l e c u l a rComposites (MICs), sol-gels, and structural nanomaterials.The presentations given at this symposium largely form thebasis for this report.

At this point in time, all of the military services and someDOE and academic laboratories have active R&D programsaimed at exploiting the unique properties of nanomaterials thath a ve potential to be used in energetic formulations foradvanced explosives and propellant applications. Figure 1 rep-resents some concepts of how nanomaterials, especially


Figure 1. Weaponization of

Advanced Energetic Technologies

Dr. Andrzej W. MiziolekWeapons and Materials Research Directorate

US Army Research Laboratory

ETC Plasma Injectors: Functionalized Polymer

Nanocomposite, Radiation Tunable

(ETC=Electro Thermal Chemical)

Novel Energetics/Nanostructured Propellant

Formulations (High Energy, Insensitive, Plasma

Specific, Low Erosivity Gradient: Nanoengineered

Progressivity, No Residue)

Novel Energetics/Nanostructured Energetics for

Warhead Explosives Formulations (High Explosive

Power, Structured Nanocomposite Matrix,

Insensitive, Environmentally Degradable)

Fuses/Initiators

(Metastable

Nanointermetallics

& Composites)

Nanocomposite Materials for Cartridge Cases

(Energetic, Consumable, Structurally Strong)

KE Munition

Thermobaric Warhead

Shaped Charge


nanoenergetics could be used for improving components ofmunitions. The figure shows that nanoenergetic compositesand ingredients can be used in the ignition, propulsion, as wellas the warhead part of the weapon. With regards to the latterapplication, nanoenergetics hold promise as useful ingredientsfor the thermobaric (TBX) and TBX-like weapons, particular-ly due to their high degree of tailorability with regards to ener-gy release and impulse management.

Metastable Intermolecular Composites (MICs)

Metastable Intermolecular Composites (MICs) are one of thefirst examples of a category of nanoscale energetic materialswhich have been studied and evaluated to a considerabledegree. MIC formulations are mixtures of nanoscale powders ofreactants that exhibit thermite (high exothermicity) behavior.As such, they differ fundamentally from more traditional ener-getics where the reactivity is based on intramolecular (not inter-molecular) properties. The MIC formulations are based onintimate mixing of the reactants on the nanometer length scale,with typical particle sizes in the tens of nanometers range (e.g.30 nm). One important characteristic of MICs is the fact thatthe rate of energy release can be tailored by varying the size ofthe components. T h ree specific MIC formulations havereceived considerable attention to date; Al/MoO3, Al/Teflon,and Al/CuO.

Research and development on MIC formulations is beingperformed in laboratories within all military services, as well asat Los Alamos National Laboratory (LANL). LANL researchersDrs. Wayne Danen and Steve Son, along with their colleagues,have not only pioneered the dynamic gas condensation methodfor the production of nanoscale aluminum powders (alsoknown as Ultra Fine Grain [UFG]), but they have also con-ducted numerous studies on physical and chemical properties.As an example, Figure 2 shows a scanning electron microscope(SEM) image of a nanoscale MIC (Al/MoO3 mixture) pro-duced by the dynamic gas condensation process at LANL. One

critical aspect of producing successful MIC formulations is theability to produce nanoscale aluminum particles of small parti-cle sizes in the tens of nanometer range, as well as with reason-ably narrow size distribution. And, of course, the productionprocess needs to be reproducible batch to batch. The currentstate of UFG aluminum production is that this is an area thatstill requires considerable effort. Even though there are com-mercial sources for UFG aluminum (such as the ALEX processoriginated in Russia, or commercial sources in other nationssuch as Japan), the need for reliable non-government sources ofingredient materials for uses in MIC applications is still there.Progress in this area is being made by companies such asTechnanogy and Nanotechnology.

Another example of a significant effort at producing MICcompounds is found at the Indian Head Division of the NavalSurface Warfare Center (NSWC/IH). This work is being per-formed by Dr. Magdy Bichay, Pam Carpenter, and TomDevendorf along with other co-workers. The Indian Headprocess for producing UFG aluminum is also based on thedynamic gas condensation process with some changes, such asthe use of resistance heating instead of RF coils. Figure 3 shows

Figure 2. SEM of MIC (Al/MoO3 Mixture)

Figure 3. The Process Schematic for the Production of Nanoscale Al for MIC

Applications at NSWC/Indian Head

Figure 4. Al Nanoparticles with a Passivation Layer of Al2O3

Al Wire

Feed

100nm 20nm

Cooling

Water

Mass

Flow

Meter

Helium

Cylinder

PyrometerElectric

Motor

Power Supply

Collection Unit

Display

Container

Vacuum

Pump

Mass Flow Meter

Oxygen

Cylinder

Power Supply

440 V, 3 Φ

SCR Thyristor

Control Unit

Vaporization

Chamber

20nm

TEM EFTEM

Al

O


the process schematic as developed by Professor Jan Puszynskiof the South Dakota School of Mines, working in concert withIndian Head. Design goals included the use of continuous alu-minum wire feed, a continuous collection system, as well as aproduction rate of approximately 10 grams/hour. It was foundthat one major limitation of the process was the 12 hour life ofthe titanium diboride/boron nitride ceramic resistance boatused for heating the material.

Figure 4 shows an example of the typical UFG aluminumthat is produced by the Indian Head process. A transmissionelectron microscope (TEM) image of the Al nanoparticles isshown on the left while an EFTEM (energy filtered TEM) isshown on the right, clearly indicating a thin passivation layer ofA l2O3. These images we re taken at Lawrence Live r m o reNational Laboratory (LLNL). In summary, much moreresearch and development needs to be done in the productionand characterization of these and new types of MIC formula-tions. Issues of MIC ignition and safety characteristics (such asimpact, friction, and electrostatic initiation) are promising, butneed to be fully explored. Overall though, certain key MICcharacteristics are very attractive and quite promising for prac-tical applications. These include energy output that is 2x thatof typical high explosives, the ability to tune the reactive power(10 KW/cc to 10 GW/cc), tunable reaction front velocities of0.1-1500 meters/sec, and reaction zone temperature exceeding3000K. Specific areas of possible applications include use inenvironmentally clean primers and detonators, chem/bio agentneutralization, improved rocket propellants, IR flares/decoys,thermal batteries, and others.

Sol-Gels

Researchers at LLNL, Drs. Randall Simpson, Alexander Gash,et al., have pioneered the use of the sol-gel method as a newway of making nanostructured composite energetic materials.The advantages of making energetics on the nanoscale areshown in Figure 5 which provides a comparison between con-ventional energetic compounds (micron scale) and those whichare composed of nanoscale ingredients. The sol-gel chemistryinvolves the reactions of chemicals in solution to produce pri-mary nanoparticles, called “sols”, which can be linked in a 3-dimensional solid network, called a “gel”, with the open poresbeing occupied by the remaining solution. There are typicallytwo types of sol-gels. “Xerogels” are the result of a controlledevaporation of the remaining solution/liquid phase, yielding adense, porous solid. On the other hand, “aerogels” can beformed by supercritical extraction (SCE), which eliminates theliquid surface tension and thus alters the capillary forces of theegressing liquid that normally would lead to pore collapse.Since the pores have been largely kept intact through the use ofthe SCE method, the resulting solid is highly porous and light-weight, with excellent uniformity given that the particles andthe pores are both in the nanometer range. Figure 6 illustratesthe sol-gel methodology.

The sol-gel approach is fundamentally different than mostapproaches to energetic material production in that it is a rela-tively simple methodology (e.g. chemistry in a beaker) per-formed at low temperatures. It can also be relatively inexpensive

and has the promise of creating entirely new energetic materi-als with desirable properties.

One current promising nanocomposite being pursued by theresearchers at LLNL involves the use of Fe2O3 which is gener-ated using the sol-gel method. The reason that Fe2O3 is chosenis because its thermite reaction with UFG aluminum is veryexothermic (with only CuO and MoO3 yielding greater energyof reaction). An example of the high degree of mixing and uni-

Figure 5. Composite Energetic Materials: Conventional vs Nanosized

Figure 6. The Sol-Gel Methodology

Figure 7. Sol-Gel Fe2O3/Al Nanocomposite

1. Conventional (µm-sized particles): • mass transport an issue

• lower power

• energy lower

(incomplete reaction)

2. Energetic nanocomposite (nm-sized particles): • mass transport minimized

• higher power

(faster reaction)

• higher total energy

• Inexpensive

• Safe

• Cast complex shapes

• Low temperature processing

Non-supercritical

extraction of

liquid phase

Condense Super-critical

extraction

Aerogel : low or high density

Xerogel: low or high density

Sol: a colloid Gel:

a 3D structure

25nm

TEM

EFTEM

Al

O

46

formity between two nanophases is found in Figure 7, whichindicates the excellent dispersion of Al and Fe on the nanoscaledomain. The Fe2O3 was prepared by the use of an organicepoxide which was added to an Fe(III) salt solution resulting inthe formation of nanoscale crystalline and amorphous Fe2O3.The reaction to produce Fe2O3 was done in solution whichalready contained the UFG aluminum. In this case, thenanoparticle aluminum was sonnicated (suspended in iso-propanol and placed in an ultrasonic bath to break up any alu-minum aggregates) before mixing with the Fe(III) salt solution.For this work, the UFG aluminum was supplied by the NSWCresearchers at Indian Head using the dynamic gas-phase con-densation method (discussed above), which yielded an averagealuminum particle size of approximately 35 nanometers.

As sol-gel materials and methodology advances, there are anumber of possible application areas that are envisioned. Theseinclude: (1) high temperature stable, non-detonable gas gener-ators, (2) adaptable flares, (3) primers, and (4) high-power,high-energy composite explosives. In addition, the sol-gelchemistry may have advantages of being more environmentallyacceptable compared to some other methods of producingenergetics.

Functionalized Carbon Nanotubes for Energetic Applications

Dr. Lalitha Ramaswamy and her colleagues at the University ofMaryland, along with Army Research Laboratory scientists(Drs. Matt Bratcher, Pamela Kaste, and Sam Trevino) areexploring the use of carbon nanotube structures (Figure 8) asstarting material for various possible energetics applications.One concept involves the functionalization of carbon nano-tubes with the notion of incorporating or binding this materi-al into propellant matrices. The hope is that by doing so therewill be significant performance enhancement in the areas of ini-tiation, overall propellant performance, safety, as well as inmechanical properties. One specific goal that is being pursuedis the assessment of carbon nanotube-based ingredients for theimprovement of propellant initiation for either an advancedplasma-based initiator (which is under development), or for usewith more conventional electrical initiators. Here the optical aswell as electrical properties of nanotubes may be important in

generating an improved propellant formulation. In particular,the high electron density that characterizes the nanotube struc-ture as well as the high conductance along the tube wall maylead to more robust and reliable ignition behavior.

Long-term storage stability is one of the key elements of asuccessful propellant formulation. In this regard, there is hopethat carbon nanotubes could be used to encapsulate nanoscaleenergetic ingredients, perhaps even the nitro-organic energeticcompounds themselves (e.g. HMX, RDX), to yield a propel-lant that not only has the same (or better) performance forenergy release, but also much improved performance for han-dling and long-term storage. An example of progress in theeffort to functionalize carbon nanotubes is given in Figure 9which shows a micrograph of some amine-terminated carbonnanotubes. In addition to the synthesis of functionalized car-bon nanotubes this effort also involves chemical analyses of theproducts as well as the use of characterization techniques suchas Prompt Gamma Activation Analysis for elemental ratioanalyses.

Center for NanoEnergetics Research at the University of

Minnesota

One of the major challenges that will be faced by theDepartment of Defense sometime in the near future (withregards to the utilization and implementation of nanoenergeticmaterials and ingredients) will be the ability to produce suchmaterials in not only large quantities, but also in controlledsizes, size distributions, and chemical compositions. To addressthis eventuality, the DOD has recently funded a university-based research program in a competitive process through theDefense University Research Initiative on NanoTechnology(DURINT) program. This research program is headed by theUniversity of Minnesota, which established the Center forNa n o Energetics Re s e a rch (CNER) (see www. m e . u m n . e d u /~mrz/cner.html). The primary goal of the CNER is to conducta comprehensive and multidisciplinary study of the high rateproduction and behavior of nanoenergetics. Professor MichaelZachariah is the Director of the CNER and he has expertise inthe synthesis and in-situ characterization of nanoparticles, aswell as molecular dynamics simulations of particle growth and

Figure 8. The Coiled

Graphitic Structure of

Carbon Nanotubes

Figure 9. Micrograph

of Amine-Terminated

Carbon Nanotubes



production. A significant aspect ofthis DURINT-funded program isthe close scientific collaborationwith a DOD research organiza-tion, namely the Army ResearchLaboratory. ARL researchers Dr.Barrie Homan and the author ofthis article are working closelywith Professors Zachariah andSteven Girshick and their researchg roups in developing virt u a l l yidentical nanoenergetics produc-tion and characterization facilities.Figure 10 shows a schematic ofthis research facility which repre-sents the centerpiece of theCNER. It is based primarily onthe use of a thermal plasma arcreactor for processing of a largenumber of possible precursors inthe solid, liquid and vapor phases.In addition to the plasma, provi-sions have been made for the useof a furnace, as well as a diffusion flame apparatus, to serve asalternate techniques for producing nanoscale ingredients. Inaddition to the production of single or multicomponent (e.g.metals or multimetals) nanoscale particles, this facility allowsfor the in-situ coating of the particles using a number of differ-ent approaches (e.g. condensation or uv curing). This coatingcould be chosen to be an inert shield to keep the core nanoin-gredient from further reacting with air (oxygen or moisture), orit could be chosen to be energetic to dramatically increase theenergy density of the final energetic formulation.

Characterizing the nanoenergetic particles produced in thisfacility is accomplished by two major diagnostic approaches.The first involves the use of a single particle mass spectrometer(SPMS, inset photo in Figure 10). Here the SPMS is used toprovide size and elemental composition of the particles as theyare sampled from the reactive flow. As such, this tool tracks thegrowth as well as the coating of the targeted nanoenergeticingredients/composites. Nanoscale particles of different sizesare selected for analysis using aerodynamic focusing. Theseselected particles can also be analyzed in-situ using spec-troscopy. Two techniques in particular are powerful tools foranalyzing small particles; Laser Induced Bre a k d ow nSpectroscopy (LIBS) and Laser Induced Incandescence (LII).LII has been developed primarily by the combustion researchcommunity as a major tool for understanding the formation ofcombustion-generated particulate matter. This technique uti-lizes a pulsed laser to rapidly heat the small particles. By mon-itoring the rate of decay of the resulting incandescent radiation,one can extract particle size information, as the rate is related tothe size of the particle. In order to get information on thechemical composition of the particles through the use of spec-troscopy, it is necessary to use a different technique. LIBS is anemerging major new tool for the analysis of chemical composi-tion (see http://www.arl.army.mil/wmrd/LIBS).

In LIBS, a pulsed laser is tightly focused on the sample toinduce a breakdown (microspark) of the sample material. Thespark process leads to the breakup of the sample into its ele-mental components and simultaneous excitation of the result-ing atoms and ions, which subsequently emit light. Thus, bymonitoring the emission from this plasma, one can determinethe nature of the element by its characteristic emission wave-length and the relative abundance by the intensity of the emit-ted light at a given wavelength. The LIBS sensor technology isa d vancing so rapidly that instrumentation has curre n t l ybecome available for the capture of light between 200-940 nm,a region where all elements emit. Thus, researchers are now ina position to simultaneously detect all of the constituent ele-ments of nanoparticles, including the metals, carbides, andorganic and/or oxide coatings. Since the LIBS data is generat-ed in real-time (response time 1 second or less), one can keeptrack of rapid changes in the composition of the particles dur-ing the actual production run. Recently Professor David W.Hahn of the University of Florida has demonstrated LIBSanalysis of nanoscale particles and has found that the LIBStechnique can be very sensitive, having a resolution in the fem-togram range (10-15 gm), and it is capable of detecting as few as100 particles per cubic centimeter.

There are a number of other researchers and topics under theCNER umbrella. Professors Steven Girshick and Sean Garrickat the University of Minnesota are working on nucleation the-ory, aerosol dynamics, and the simulation of reactive flowswhile their colleague Pro f. Alon McCormick, as well asProfessor Tom Brill of the University of Delaware, are workingon various aspects of sol-gel science. Other principal investiga-tors include Professor Jan Puszynski of the South DakotaSchool of Mines who is working on solid-state chemistry andkinetics. On the theoretical side, Professors Don Thompson ofOklahoma State University and Don Truhlar of University of

Figure 10. Schematic of Instrument to Produce Nanoscale Energetic Ingredients (Photo inset

is the single particle mass spectrometer at the CNER- University of Minnesota)

Advanced Nanoenergetic Ingredients for Explosives & Propellants

Nanometals, Nanoorganics, Coatings; Production, Characterization, Processing

Energetic

Organics

and Solvent

or Sol-Gel

Agile research facility for the production & properties optimization of nanoenergetic materials.

Coating Flow

Counter Flow

Thermal Plasma Reactor

Flame Furnace Particle

Collector

Pump

Flow

“Spray-Freeze”

Coating Reactor

UVSource

48

Minnesota are working in the area of fundamental computa-tions aimed tow a rds the simulation of the behavior ofnanocomposites. All in all, the CNER represents a majornational resource for understanding the science and engineer-ing as they relate to the generation and behavior of nanoener-getic compounds and ingredients. It should be mentioned thatthe Army Research Office (Drs. David Mann and RobertShaw) had a major role in the selection of the DURINT topicon nanoenergetics and in the continued administration of theCNER contract which started in 2001 and could run through2005.

Acknowledgements

The author would like to thank Drs. Alex Gash, Jan Puszynski,Lalitha Ramaswamy, Steven Son, and Michael Zachariah forfurnishing information and for their assistance in producingthis article. Also, I would like to thank my ARL colleagues Drs.Brad Forch, Barrie Homan, Pamela Kaste, and Rose Pesce-Rodriguez for their editorial assistance.

Selected References

Aumann, C.E.; Skofronick, G.L.; and Martin, J.A., “OxidationBehavior of Aluminum Nanopowders,” Journal of VacuumScience & Technology B, Vol. 13(2): pp. 1178-1183 (1995).

Bratcher, M.; Pesce-Rodriguez, R.; Kaste, P.; Ramaswamy, A.L.,“Nanotube Modification of Energetic Materials”, Proceedings ofthe 38th Meeting of the JANNAF Combustion Subcommittee,Destin, FL, April (2002).

Cliff, M.; Tepper, F.; Lisetsky, V., “Ageing Characteristics ofAlex Nanosize Aluminum,” AIAA-2001, p. 3287 (2001).

Gash, A.E.; Simpson, R.L.; Tillotson, T.M.; Satcher, J.H., andHrubesh, L.W., “Making Nanostructured Pyrotechnics In ABe a k e r,” Proceedings of the 27th In t e rnational Py ro t e c h n i c sSeminar, pp. 41-53, Grand Junction, Colorado (2000).

Gash, A.E., Tillotson, T.M.; Satcher, J.H., Jr.; Hrubesh, L.W.;Simpson, R.L., “Use of Epoxides in the Sol-Gel Synthesis ofPorous Fe2O3 Monoliths from Fe(III) Salts”, Chem. Mater.2001, Vol. 13, p. 999, (2001).

Gash, A.E., Tillotson, T.M.; Poco, J.F.; Satcher, J.H., Jr.;Hrubesh, L.W.; Simpson, R.L., “New Sol-Gel Synthetic Routeto Transition and Main-Group Metal Oxide Aerogels UsingInorganic Salt Precursors”, J. Non-Cryst. Solids, Vol. 285, pp.22-28, (2001).

Hahn, D.W. and Lunden, M.M., “Detection and Analysis ofAerosol Particles by Laser-Induced Breakdown Spectroscopy”,Aerosol Science and Technology, Vol. 33, p. 30-48 (2000).

Ma rtin, J.A.; Mu r r a y, A.S.; and Busse, J.R., “Me t a s t a b l eIntermolecular Composites”, Warhead Technology, pp. 179-191(1998).

Miziolek, A.W.; McNesby, K.L.; and Russell, R.S., “MilitaryApplications of Laser Induced Bre a k d own Sp e c t ro s c o p y(LIBS)”, Abstract Book for Pittcon 2002, New Orleans, LA,March (2002).

Puszynski, J.A., “Advances in the Formation of Metallic andCeramic Nanopowders”, Powder Materials: Current Researchand Industrial Practices, pp 89-105, F.D.S. Marquis ed., TheMinerals, Metals, and Materials Society (2001).

Puszynski, J.A.; Jayaraman, S.; Bichay, M.; Carpenter, G., andC a r p e n t e r, P., “Formation and Reactivity of Na n o s i ze dAluminum Powders”, Proceedings of the World Congress onParticle Technology, Sydney, Australia, July (2002).

R a m a s w a m y, A.L. and Kaste, P., “Nanoscale Studies forEnvironmentally Benign Explosives & Propellants”, Proceedingsof the Meeting on Advances in Rocket Propellant Performance,Denmark, September (2002).

Simpson, R.L.; Tillotson, T.M.; Satcher, J.H., Jr.; Hrubesh,L . W.; Gash, A.E., “Na n o s t ru c t u red Energetic Ma t e r i a l sDerived from Sol-Gel Chemistry”, Int. Annu. Conf. ICT (31stEnergetic Materials), Karlsruhe, Germany, June, (2000).

Son, S.F.; Asay, B.W.; Busse, J.R.; Jorgensen, B.S.; Bockmon,B., and Pa n t oya, M., “Reaction Propagation Physics ofAl/MoO3 Nanocomposite Thermites”, Proceedings of the 28thIn t e rnational Py rotechnics Se m i n a r, pp. 833-843, Ad e l a i d e ,Australia, November (2001).

Tillotson, T.M.; Gash A.E.; Simpson, R.L.; Hrubesh, L.W.;Thomas, I.M.; Poco, J.F., “Nanostructured Energetic MaterialsUsing Sol-Gel Methods” J. Non-Cryst. Solids, Vol. 285, pp.338-345, (2001).

Zachariah, M.R.; Mehadevan, R.; Lee, D.; and Sakurai, H.,“Measurement of Solid State Reaction Kinetics in the AerosolPhase Using Single Pa rticle Mass Sp e c t ro m e t ry”, De f e n s eApplications of Na n o m a t e r i a l s, Miziolek, A.W.; Karna, S.;Mauro, M.; Vaia, R., eds., ACS Symposium Series Book,American Chemical Society, Washington, DC (in preparation -to be published in 2002).

Dr. Andrzej W. Miziolek is a Research Physicist at the US Army Research Laboratory/Weapons and Materials Research

Directorate, Aberdeen Proving Ground, MD. He is recognized for his expertise in combustion (particularly flame suppression) and

plasma research; multiphoton spectroscopy and collisional dynamics; and for his work in applying laser spectroscopy to problems

in chemical analysis and combustion diagnostics. Recently he has focussed his attention to research on nanoscale materials, in

particular on nanoenergetics, as well as on developing technology for force protection and anti-terrorism applications. Dr.

Miziolek has co-authored over 45 refereed journal papers, three book chapters, and 80 government technical reports and

publications. He has published a book on Halon Replacements: Technology and Science (A.W. Miziolek and W. Tsang, eds.,

ACS Symposium Series 611, 1995) and is currently working as lead editor on two books: (1) Laser Induced Breakdown

Spectroscopy (LIBS): Fundamentals and Applications to be published by Cambridge University Press, and (2) Defense

Applications of Nanomaterials to be published by ACS Books.



Sensors are not the first things that come to mind when con-sidering the panorama of potential applications anticipatedfrom the burgeoning field of nanoscience and nanotechnology.However, sensors are ubiquitous and play critical roles in thefunction of a wide range of electrical and electronic systems,from tiny electronic circuits to large and complex systems. Forinstance, electronic and optoelectronic circuits need voltage,current, temperature, light, and other sensors to operate.Jumbo jets need mechanical sensors and actuators to function.Nanotechnolgy offers hitherto unheard of possibilities in revo-lutionizing the sensors in use today and opening new frontiersin functionalities. To paraphrase Richard Feynman (please seethis issue’s MaterialEASE), there is indeed still much room at thebottom [1].

The Sensors and Electron Devices Directorate of the USArmy Re s e a rch Laboratory (ARL/SEDD), located in theMaryland suburbs of Washington, DC, is dedicated to thedevelopment of state-of-the-art sensors for our warfighters. Weare actively pursuing avenues that take advantage of the excit-ing potential of nanosystems to improve on the currentapproaches, as well as look ahead to new sensor modalities. Forexample, we are harnessing the properties of semiconductorquantum-dots (QD) to improve infrared detection and are pur-suing nanosize gold (Au) colloidal particles to enhance detec-tion of biological threats. The advantages of nanoengineeredmaterials are realized by harnessing the quantum propertiesobservable in the nanoscale regime. Nanoscaled objects com-prise the building blocks of the mesoscale world as we know it.However, our technology has only recently allowed us to designstructures at this scale by having fundamental control overphysical and chemical processes and molecular attributes.

ARL/SEDD has taken an integrated approach to developingsensors and advancing the underlying technology. For example,nanophase materials are grown and fabricated through tech-niques such as molecular beam epitaxy (MBE), yielding semi-conductor quantum dots for infrared (IR) detectors. Thesematerials are fabricated to produce functional devices which arethen packaged and integrated into subsystems to produce fullyfunctional prototype sensors. Substantial facilities, equipmentand investments have been dedicated to the emerging field ofnanosensors.

The Army has developed specialized tools to fabricate andanalyze these tiny structures. One example is the magnetic res-onance force microscope (MRFM), which is a scanning-probe-type instrument capable in some instances, of measuring thenuclear magnetic resonance (NMR) response of a single pro-ton. Instead of detecting small electrical signals as convention-al magnetic resonance imaging (MRI) does, MRFM detectsatomic and subatomic forces mechanically. The instrumentuses tiny magnets mounted on cantilevers fabricated usingMEMS techniques. MRFM is several orders of magnitudemore sensitive than conventional MRI for objects smaller thanone-thousand cubic micrometers. Until recently, conventionalelectrically-detected MRI has lacked the sensitivity to makesolids MRI useful, but MRFM will make force detected solidsMRI a reality. (Note: Solids MRI differs significantly frommedical MRI, which is a form of liquids MRI. This is becausethe human body is primarily composed of water, which pro-vides an excellent NMR response to traditional magnetic reso-nance technology.) When perfected, MRFM should be able toimage a single atom in a three dimensional (3-D), non-homog-enous object. Such a breakthrough would find broad applica-tions in biology, medicine, materials science, semiconductors,polymers, and many other areas.

Self-Assembled Quantum Dots for IR Detector Application

Molecular beam epitaxy (MBE) is a specialized, ultrahigh vac-uum evaporation technique normally used to grow multilayersemiconductor films with interfaces that are essentially flat,even at the atomic level. However, if a material with a relative-ly large lattice constant were grown on top of a material with asmaller lattice constant, the large compressive strain energycould be relieved by spontaneous coalescence of the larger lat-tice constant material into 3-D ‘islands’ on the surface. Theseislands would take the form of crystallographic pyramids ormultifaceted domes and could range in size from a fewnanometers to several hundred nanometers. Because of theirextremely small size and the manner in which they are formed,they are referred to as self-assembled quantum dots.

Figure 1 shows an atomic force micrograph of a set of InAsquantum dots grown on a GaAs substrate. The lateral dimen-sions are exaggerated because the radius of curvature of the

Dr. Paul M. Amirtharaj, Dr. John Little, Dr. Gary L. Wood, Dr. Alma Wickenden, and Dr. Doran D. SmithSensors and Electron Devices Directorate

US Army Research LaboratoryAdelphi, MD

probe tip is comparable to the size of the dots. If the islands areembedded in a host material with a larger bandgap (e.g., bycrystal overgrowth of GaAs), they form isolated 3-D potentialwells that have been shown to bind several discrete energy lev-els much like isolated atoms [2]. This is the 3-D analog to theone-dimensional (1-D) semiconductor quantum well that isformed when a very thin layer of a low-bandgap material issandwiched between two higher-bandgap layers.

One of the results of carrier confinement in quantum wells isa strong infrared absorption that occurs when electrons in theground state of the well (due to doping with electron donorimpurities) are excited to a higher energy subband (allowedenergy level within the conduction and valence “bands” of thequantum well). This effect has been used to make quantumwell infrared photodetectors (QWIPs), which are the funda-mental element of high-performance thermal imaging systems.Polarization selection rules for these intersubband transitionsdictate that only light that is polarized in the direction of quan-tization (i.e., perpendicular to the quantum well layer) can beabsorbed. Unfortunately, the typical geometry of a 2-D imag-ing array of QWIPs is such that light incident on the array hasno polarization component perpendicular to the quantum welllayers. This requires that each pixel of the array have a polar-ization rotating structure (typically a diffraction grating) etchedinto its surface, which limits the efficiency and producibility ofQWIP detector arrays.

Since quantum dots confine carriers in all three dimensions,intersubband transitions can be excited by all polarizations ofinfrared light. Also, due to the discrete nature of the energy lev-els, the peak oscillator strength for normal-incidence infraredabsorption should be very large in doped quantum dots. Thissuggests that quantum dots could be used to replace integratedoptical coupling structures used in quantum well infrareddetectors.

Figure 2 shows the polarization-dependent infrared pho-tocurrent spectra of a quantum dot infrared photodetector(QDIP), consisting of a stack of InAs quantum dot layers sep-arated by GaAs barrier layers. The QDIP responds to bothpolarizations of light in the wavelength range of interest for

thermal imaging. However, infrared detectors based on quan-tum dots are in their earliest stages of development. The darkcurrents are high, and it is difficult to predict the operatingwavelength a priori because of the complexity of the structures.Researchers at ARL are currently conducting a systematic studyof the interactions between the electronic levels of the quantumdots and the quasi-continuum of electronic states in the host(barrier) material surrounding them. Because of the wide rangeof semiconductor materials that are available for use in quan-tum dot systems and the flexibility and control of growthparameters offered by MBE techniques, rapid advances in theperformance of infrared detectors based on self-assembledquantum dots will occur in the near future.

Nanotechnology for Displays and Biodetection

The Army has extensively investigated nanoscale structures toenhance the performance of a number of engineered materialsystems for a variety of sensor applications. Dendrimers havebeen examined as a means to encapsulate or capture chro-mophores and augment their physical and chemical properties.In other work, nanoscale structures and particles have beeninvestigated for luminescent research that could have applica-tions for future Army displays. Finally, nanostructures are beingi n vestigated for surface-enhanced Raman scattering in anattempt to detect and identify harmful biological agents in theenvironment.

Re s e a rch and development of electronic materials form thefoundation of the modern information re volution. As thedimensions of bulk solid-state devices rapidly approach thelimits of pro c e s s a b l i t y, molecular-based devices that inher-ently take advantage of quantum physics will play incre a s-ingly important roles in electronic evolution. Organic semi-conductors have demonstrated their potential as a new classof electronic materials for commercial applications including

50

Figure 2. Photocurrent Spectra for a InAs/GaAs

Quantum Dot Detector Structure (grown, fabricated,

and tested at ARL) for Light Polarized Parallel and

Perpendicular to the Quantum-dot Layers

9

8

7

6

5

4

3

2

1

0

2 4 6 8 10 12 14

E vector parallel to dot layers

E vector perpendicular to dot layers

Wavelength (µm)

Figure 1. Atomic Force Micrograph of InAs

Quantum Dots on a GaAs Substrate

0.0 0.25 0.50 0.75 1.00

micrometers


1.00

0.75

050

0.25

0.00

e l e c t ro p h o t o g r a p h y, light-emittingdiodes (LEDs) [3], transistors [4],and sensors [5]. The pioneeringw o rk by Tang and Van Slyke [3]demonstrated that organic light-emitting devices (OLEDs) indeedh a ve the necessary attributes andc o m p e t i t i ve advantages to be con-s i d e red seriously for display appli-c a t i o n s .

An important distinction must bemade between inorganic semicon-ductors and organic-based semicon-ductors. Inorganic semiconductorsh a ve occupied and unoccupiedenergy bands, valence, and conduc-tion bands, respectively, that canextend over many unit cells. Fortypical Si-based semiconductors,these energy bands are determinedby a tetrahedral bonding structureformed by sp3 hybridization. Fororganic semiconductors, the occu-pied and unoccupied energy levelsare formed from planar structures ofsp2 bonds as well as π-bonds. Thecombination of bonds form thehighest occupied molecular orbitals (HOMO) and lowestunoccupied molecular orbitals (LUMO), where the benzenering is the typical bonding structure for most organic semicon-ductors.

For OLED applications, the organic films are typically notcrystalline, which leads to a high degree of localization of themolecular orbitals on a single molecule. Thus, molecular-basedelectro-optical devices such as OLEDs rely on nanotechnology-

engineered materials that take advantage of quantum confine-ment at a molecular level. In OLEDs, the transport and lightemission properties are limited by the injection and subsequentdiffusion of hole and electron carriers from the anode and cath-ode, respectively, to the light emission region. The drift diffu-sion of holes and electrons is determined by a field-dependenthopping from a molecular orbital state that is energeticallylower than the molecular state in a neighboring molecule.

Cross-section of layered thin-film structure

LUMO

Energy

Levels

Photon

AlqNPB

–

++

+

–

–

HOMO

Energy

Levels

Electron migration

Electron-Hole pair forms

Excition, produces photon

Hole migration


Figure 3. Energy Diagram for a Typical Small Molecule OLED

Individual Fit Results

Experimental Data

NPB

Temperature (K)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Fit Results

Experimental Data

Alq

Temperature (K)

NPBAlq

Figure 4. Thermally Stimulated Luminescence (TSL) Spectra for NPB and Alq. Results Show that the Energy Disorder (corresponding to

temperature in the graphs) in Alq is Twice as Large as NPB. (The TSL for Alq occurs at a higher temperature than it does for NPB, thus

the greater energy disorder.)

0 50 100 150 200 250 3000 100 200 300

52

Then, an electron-hole pair combines on a single molecule,forming an exciton that ultimately decays with the emission ofa photon.

Figure 3 displays an energy diagram for a typical small mol-ecule OLED. The energetic disorder in molecular glassy filmshas been measured by the Army using thermally stimulatedluminescence (TSL) [6]. These results show that the energy dis-order is inherently larger in the electron-transporting tris-8hydroxyquinoline aluminum (Alq) layer as compared to thehole-transporting N,N'-bis-(1-naphthyl)-N,N'-diphenyl1-1,1-biphenyl1-4,4'-diamine (NPB) layer. This is in good agreementwith the lower electron mobility in Alq as compared to NPBand can lead to an unbalanced carrier pair in the emissiveregion and device instabilities [7]. Figure 4 compares the TSLfor NPB and Alq. In addition to organic-based displays, we areinvestigating inorganic nanoparticle-based emitters for displayapplications. As with organics, the quantum confinement innanophosphors improves the recombination cross-section andlight output efficiency.

Raman spectroscopy is an optical technique that providesinformation about the structure of molecules. It can be used inaqueous systems due to the weak Raman signal from water,making it ideal for analysis of biological organisms. Ramanspectroscopy is inherently weak, but methods to enhance thesignals have been known for over 25 years. Surface-enhancedRaman spectroscopy (SERS) requires nanometer-scale metalfeatures to enhance the signals by up to 16 orders of magni-tude. Army researchers are investigating several methods forpreparing substrates with such nanoscopic features for thedetection and identification of bacteria [8]. They include Aucolloidal particles immobilized on surfaces, electrochemicallyroughened Au surfaces, and periodic particle arrays from Prof.Richard van Duyne at Northwestern University [9]. In each ofthese substrates, the feature size is in the 10 nm to 150 nmrange. The degree of enhancement is strongly related to the sizeof the nanoscopic features and the excitation wavelength used.

The first critical step in collecting SERS spectra of bacteria isto develop an SERS substrate that the bacteria can approachwithin 10-100 nm. This distance requirement is established bythe short range of the SERS enhancement. Since both a metalsurface or particle and the bacterium have a negative surfacecharge, it is necessary to either modify the particle or surfacewith a small molecule to neutralize its charge or hold the sur-face at a neutral or slightly positive charge. The size of theenhancing features can then be optimized for the chosen exci-tation wavelength. Once the bacteria move within the SERSenhancement range, the spectra can be collected using laserexcitation at 647.1 nm. Once a library of spectra is collected,advanced statistical analysis methods such as chemometrics ora neural network can be trained for the identification of thegenus and species of the bacterium.

Nanoscale Electromechanical Systems (NEMS)

for Radio frequency (RF) Filter Applications

ARL has a strong effort in piezoelectric microelectromechanicalsystems (MEMS) device fabrication, with specific expertise inlead zirconate titanate (PZT) based piezoelectric devices. These

include thin film resonators, magnetometers, vacuum pumpsfor chip-scale mass spectrometers, and power generationdevices. PZT resonators with resonant frequencies from 0.2 to9 MHz have been fabricated at ARL, and their response hasbeen studied intensively [10]. Resonator arrays for RF filterdevices operating in the GHz frequency range are of interest tothe Army for lightweight, low-power, high-precision frequencyselection applications, including wireless communication,tracking and surveillance, and weapons guidance and fusing. Ahigh quality factor, or Q, is required in the resonator device toreduce phase noise and ensure stability against frequency-shift-ing phenomena [11].

Electromechanical resonator devices offer potential advan-tages in size, weight, and power consumption over surfaceacoustic wave (SAW) or bulk acoustic resonators currently usedfor frequency filtering applications. Pi ezoelectric electro-mechanical resonator devices should demonstrate advantagesover equivalent electrostatic devices for high-frequency applica-tions, since they are less sensitive to degraded coupling strengthas the device dimensions are reduced [12]. Equation 1 displaysthe Euler-Bernoulli beam equation for the natural resonancefrequency of a doubly clamped mechanical resonating beam,where L is the length of the beam, t is the beam thickness, E isthe elastic modulus, and ρ is the density [13].

Ω0(L) E t——— = 1.027 — — Eq. 1

2π ρ L2

For resonance frequencies in the GHz range of interest,electromechanical resonator devices typically require lengths inthe range of 1 to 10 mm, with thickness and width dimensionson the order of hundreds of nanometers. Material propertiesfactor prominently in the frequency response. The roles ofgrain boundaries, defects, and other nonbulk materials charac-teristics of thin films are unknown at present and must beinvestigated to effectively model device response and to ensurepredicted device operation and reliability.

Research on NEMS for high-Q resonator filter array fabrica-tion has recently begun at ARL, in collaboration with researchgroups at the University of Maryland. While PZT is an attrac-tive material for piezoelectric filters (due to its high piezoelec-tric coupling coefficient [14]), the operating frequency of PZTresonators is limited to the MHz range, due to the largeacoustic time constant of the material. For this reason, piezo-electric materials such as aluminum nitride (AlN), zinc oxide(ZnO), and related compounds are being investigated. The fastacoustic response times of these wide bandgap semiconductormaterials translate to theoretical maximum frequencies of morethan 100 GHz [15]. Additionally, AlN and ZnO may be grownas thin films (100 nm thick or less) by a variety of techniques,on either silicon substrates (for cost advantages) or a widebandgap substrate such as silicon carbide (SiC). AlN in partic-ular is chemically inert, radiation hard, and can withstand hightemperatures, and chemically harsh environments, making itan excellent candidate for integration of NEMS resonator fil-ters into SiC integrated circuits.


√


Detailed device models are required to ultimately enhance fil-ter performance and sensor output quality. The investigation ofnonlinear behavior and nanoscale surface effects is plannedusing ARL’s characterization capabilities described elsewhere inthis article. In addition, the formation of thin film microstruc-ture is being investigated in collaboration with the CaliforniaInstitute of Technology (Caltech.) Predictive theory, validatedby targeted experiments, is being developed to guide materialsdevelopment, materials selection, and device design. In partic-ular, the piezoelectric materials used as the active componentsof the NEMS resonator devices are typically composed ofdomains due either to the deposition technique or to therequirement of unmatched materials systems in the devicedesign. The effects of domain structure in the thin films, wherethe film thickness may be less than or of the same approximatescale of the domains themselves, is considerably different fromthat of bulk polycrystalline or textured materials. The resultantdevice properties may be strongly influenced by the ability tomanipulate the domain structure. In the ideal case, a resonatorbeam formed on a single grain would be expected to exhibitsingle crystal properties that would result in maximum Q forthe device.

The fabrication of MEMS and NEMS devices is performedin ARL’s new state-of-the-art 10,000 square foot class 100Cleanroom. An expansion of this facility is currently beingplanned to include additional Class 100 and Class 10 fabrica-tion areas. A compliment of pattern transfer techniques areavailable for MEMS/NEMS device fabrication, as listed in

Table 1. Electron beam direct-writelithography is used to generate devicef e a t u res on the scale of tens ofnanometers. Figure 5 illustrates 50nm scale quantum dot patterns with100 nm spacing exposed in poly-m e t h y l m e t h a c rylate (PMMA) elec-t ron beam resist, and gold fingerstructures with widths of less than 40nm, which have been patterned inA R L’s fabrication facility. Si m i l a rmetal patterns are used as maskingl a yers for subsequent re a c t i ve ionetching or ion milling techniques,which enable pattern transfer intothin films that form the active devices t ru c t u re. Both anisotropic andisotropic dry etching techniques areemployed in the fabrication of free-standing MEMS/NEMS resonators.In addition to developing prototypedevices, test structures are speciallyfabricated to investigate the physicalp ro p e rties of thin film materials.These structures provide feedback tothe fundamental materials investiga-tions on grain formation and alsofacilitate direct measurement of phys-ical properties such as tensile strength

of thin films formed under varying deposition or growth con-ditions. Such fundamental information will subsequently beused to allow more accurate and sophisticated modeling ofdevices, enabling the improvements in device operation anddesign through investigation of materials and device propertiesat the nanoscale.

Force Detected Magnetic Resonance of GaAs

Force detected magnetic resonance is a recent technique for thedetection of magnetic resonance, where the signal is detectedmechanically instead of electrically. Magnetic Re s o n a n c eImaging (MRI) is the most recognizable nuclear magnetic res-onance (NMR) application, as its primary use is for medicalpurposes. Conventional MRI employs electrical detection,effectively for most objects greater than 10 mm in size.Electrically detecting NMR is very insensitive at smaller scaleswhile mechanical detection on the other hand, is much moresensitive at the nano and low-micron scales. The first proposaland demonstration of a technique for mechanically detectingmagnetic resonance was done by A. Gozzini in 1963 [16]. Atthe time the approach was not practical but research continued.In 1991, John Sidles [17, 18] suggested attaching a small mag-netic particle to a cantilever and using the magnetic field gra-dient from the particle to perform nanometer imaging. Sidlescalled his approach magnetic resonance force micro s c o p y(MRFM).

MRFM entails measuring the force between the magneticmoment of nuclei (or electrons) in a sample and the magnetic

Figure 5. Scanning

Electron Microscopy

(SEM) Images of

Patterns Written at ARL

using Electron-Beam

Direct-Write Lithography:

a) 50nm Quantum Dot

Patterns on 100 nm

Centers, and b)

Nanoscale Gold Finger

Structure, with Metal

Deposited using

a Liftoff Technique

after Lithographic

Patterning

(a)

(b)

206nm

37nm

54

moment of a nearby magnetic particle (Figure 6). A magneticparticle is attached to a microcantilever, and the sample isplaced nearby. The sample contains nuclei that have an associ-ated magnetic moment. The magnet mounted on the cantileverfeels a force from the sample, thus displacing the cantilever.The two magnets either attract or repel each other dependingon their relative orientation.

By using a combination of amplitude and frequency-modu-lated RF radiation the sample magnetic moments are cyclically‘flipped’. Cyclical flipping of the magnetic moments, or mag-netic resonance, occurs when the energy of an individual RFphoton (energy = Planck constant times frequency, or E=hf)equals the energy difference between the nuclei’s energy levels.Each nuclei has energy levels that are due to the relative orien-tation of its magnetic moment with respect to a backgroundmagnetic field (parallel or antiparallel). For example, magneticresonance for 75As (Arsenic-75 isotope) at 1 T (Tesla) occurs ata frequency of 7.2919 MHz and is 51.56 MHz at 7.07 T. Thedifference between the energy levels is a linear function of the

magnetic field stre n g t h .When the RF is in resonancewith the energy differe n c e ,the nuclei either absorb ener-gy or are stimulated to emite n e r g y, there by oscillatingbetween the two energy lev-els. The nuclei’s oscillationbetween energy levels oscil-lates the nuclei’s magneticmoment between parallel andantiparallel to the magneticfield.

As the two magnets cycli-cally attract and repel eacho t h e r, the cantilever willbegin to oscillate. This oscil-lation is detected by a fiber-based interferometer. If Q isthe mechanical quality factorof the cantilever, applying anoscillating force at the

mechanical resonance frequency of the cantilever results in a Qenhancement of the cantilever’s amplitude. Thus, the signal ismechanically amplified at least 10,000 times, making detectionwith the fiber interferometer easier.

A number of groups have reported force-detected NMR insolids. Rugar was the first to report an NMR signal usingMRFM techniques [19], exhibiting an orders-of-magnitudeimprovement over conventional NMR. The magnetic particlecreates a changing magnetic field across the sample that assuresthat only nuclei within a narrow slice experience the correctvalue of magnetic field to be in resonance with the RF. Nucleioutside of the resonant slice are at the wrong magnetic field tobe in resonance with the RF. The slice-selective character ofMRFM allows a 3-D image of the sample to be obtained byscanning the magnetic particle with respect to the sample in3-D [20].

The proximate goal of our research is force detection andimaging of nuclear magnetization in as-grown material and inas-fabricated Group III-V (of the periodic table) semiconduc-


Figure 6. Typical MRFM Apparatus

Cantilever

Magnetic Particle

RF Coil

Resonant Slice

Sample

Optical Fiber

E-beam lithographyContact lithographyPhotolithography cluster toolUV photostabilizationDownstream plasma ashingMask makingSilicon deep reactive ion etching

(DRIE)Isotropic silicon etching Deep dielectric etchingRefractory metal etching

Dielectric (oxide/nitride) dry etchingIon millingPZT reactive ion etchingLaser Doppler vibrometryMetal sputteringMetal/compound e-beam evaporationPlasma Enhanced Chemical Vapor

Deposition (PECVD) oxide/nitrideLow Pressure CVD oxide/nitrideSol-gel PZTWet and dry thermal oxidation

Wafer-to-wafer alignmentAnodic and fusion wafer bondingVariable angle spectroscopic

ellipsometryWafer bow stress measurementNanospec film thickness analysisFour point probe analysisProfilometery

Table 1. Partial List of Capabilities for MEMS/NEMS Device Processing in ARL’s Class 100 Cleanroom Facility


tor devices. As a step toward this goal, we have constructed anMRFM apparatus suitable for force detecting NMR at 4 K. Weapplied the technique to observe 69Ga, 71Ga, and 75As in GaAs,increasing the number of isotopes observed with force detec-tion from four (1H, 19F, 59Co, and 23Na) to seven [21]. The sam-ple we examined was a 3-µm epilayer of GaAs, doped at 0.6 x1018 cm-3 Si and 2.0 x 1018 cm-3 Be. A hand-cleaved rectangle ofthis material (~210 µm x ~150 µm) was then attached to thecantilever (k ~0.05 N/m) using silver-filled epoxy. This can-tilever was coated with 0.2 µm of Au on each side to enhancethermal conductivity, and when loaded it had a resonance fre-quency of 1.034 KHz, and Q = 154 at 5 K in the presence ofHe exchange gas. Q is the quality factor of the cantilever reso-nance. The magnetic gradient source was a 250 µm diameter Fecylinder positioned ~35 µm from the sample. Because ofNewton’s third law of motion – for every force there is an equaland opposite force - it does not matter if the sample or mag-netic particle is attached to the cantilever. A 1 1/2 turn, 710-µm diameter coil generates the 4-gauss RF magnetic field. Themotion of the cantilever was detected by a wavelength-tunedfiber optic interferometer.

Figure 7 displays a force-detected NMR spectrum of oursample observed by sweeping the applied magnetic field at afixed RF center frequency of 51.56 MHz. All three isotopes areclearly resolved, with cantilever displacements of ~1.3 picome-ters. The large width of the NMR peaks represents the spatialextent of the sample.

As shown, Army researchers have applied force-detectedNMR techniques to GaAs and imaged its nuclear isotopes with1-µm resolution. MRFM technology promises countless appli-cations to material science, medicine, biology, and device fabri-

cation. The ultimate goal for the MRFM community is 3-Dimaging of proteins, where the location of every proton in theprotein is measured with 0.1-nm accuracy (protein structure).For Group III-V device work, ARL expects to be able to meas-ure such properties as the local value of the electric field; theshape of electron wavefunctions in quantum wells; and 3Dstrain, internally and nondestructively in as-grown material andas-fabricated devices.

Conclusion

The potential of nanostructures seems limitless in providingdetailed information on the cellular, molecular and even atom-ic level processes. Substantial advantages have already been real-ized in radiation detection as well as electronic displays.Revolutionary detection modalities, such as real-time biologicalthreat detection, are on the horizon. The future undoubtedlywill bring more exciting developments which will advance thestate-of-the-art and in the process, provide our military servic-es an additional edge over our adversaries.

Acknowledgements

The authors wish to acknowledge the contributions made by the fol-lowing persons to this article: Eric Forsythe, Nicholas F. Fell, Jr., BrettPiekerski, and Kent R. Thurber.

References

[1] R.P. Feynman, There’s Plenty of Room at the Bottom, Reprinted inJMEMS 1, 60 (1992).[2] M. Grunmann, O. Stier, and D. Bimberg, Phys. Rev. B15, 52 (16),11969 (1995).[3] C.W. Tang and S.A. VanSlyke, Appl. Phys. Lett. 51, 913 (1987).

[4] D.J. Gundlach, Y.Y. Lin, T.N.Jackson, S.F. Nelson, and D.G.Schlom, IEEE Elec. Dev. Lett., 18,87 (1997).[5] Q. Zhou and T.M. Swager, J.Amer. Chem. Soc. 117, 7017 (1995).[6] E.W. Forsythe, D.C. Mo rt o n ,C.W. Tang, and Y. Gao, Appl. Phys.Lett. 73, 1457 (1998).[7] H. Aziz, Z.D. Popovic, N.X. Hu,A.M. Hor, and G. Xu, Science 283,1900 (1999).[8] N.F. Fell, Jr., A.G.B. Smith, M.Vellone, A.W. Fountain III, SPIE Vol4577 Bellingham, WA, in pre s s(2001).[9] C.L. Haynes and R.P. van Duyne,J. Phys. Chem. B 105, 5599-5611(2001).[10] B. Piekarski, D. De Voe, M.Du b e y, R. Kaul, and J. Conrad,Sensors and Actuators A 91, 313-320(2001).[11] C.T.-C. Nguyen, L.P.B. Katehi,and G.M. Rebeiz, Proc. IEEE, Vol.

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Applied Magnetic Field (Tesla)

71Ga 69Ga 75As

Figure 7. Typical Plot Output for an MRFM

56

Dr. Paul M. Amirtharaj is a Physicist at ARL with expertise in the interaction of light with semiconductors. His current research inter-

ests include the physics of nanoscale structures and devices, in particular their optical behavior. He has held staff positions at the

Night Vision Lab (1984-1989), National Institute of Standards and Technology (1990 to 1999) and joined the Sensors and

Electron Devices Directorate at ARL in 1999. He has coauthored more than 60 publications which include three book chapters.

Dr. Amirtharaj received his Ph.D. from Emory University in 1981.

Dr. John W. Little received his Ph.D. in physics from the University of Tennessee in 1982. His thesis work involved theoretical and

experimental studies of the optical properties of metallic nanoparticles. He currently leads a research team in the Infrared

Materials Branch at the Army Research Laboratory in Adelphi, Maryland. His work includes studies of III-V semiconductor struc-

tures such as strained-layer superlattices, quantum wells, and quantum dots for application in high-performance thermal imaging

systems.

Dr. Doran D. Smith, received his Ph.D. in physics from the University of Michigan in 1985. He joined the Army Research Laboratory

(then LABCOM) in 1985 where he worked on mesoscopic devices, magneto-transport and magneto-optical properties of III-V

semiconductors, and high-speed devices. In 1995 he initiated ARL’s force detected magnetic resonance program. Dr. Doran holds

two patents and is the author of 35 publications and one book chapter on single electron devices.

Dr. Alma E. Wickenden conducts research in MEMS/NEMS devices and process development in the Sensors and Electronics

Devices Directorate at ARL. Her research interests focus on the relationship between the microstructure of materials and resultant

electronic and mechanical properties, and she has published over 60 papers in the field. She is currently pursuing electron beam

lithography of nanoscale electromechanical devices and novel sensor concepts. From 1993-2001, she conducted research on

III-nitride semiconductor thin film growth by MOCVD at the Naval Research Laboratory. She had previously worked as an engi -

neer in industry in GaAs VPE growth and SAW device process development. Dr. Wickenden received her Ph.D. (1993) and M.S.

(1989) degrees in Materials Science and Engineering from Johns Hopkins University, Baltimore, MD.

Dr. Gary Wood received his B.S. in Physics from Drexel University in 1980, his M.S. in Physics in 1982, and his Ph.D. in Physics

from Catholic University In 1992. Dr. Wood has been a US Army Civilian Employee from 1982 to the present. He has worked

at both the Night Vision Lab and the Army Research Laboratory over that period. Dr. Wood’s research has included basic and

applied research in the areas of nonlinear transmission and light induced gratings. Dr. Wood is currently the Branch Chief of the

Optics branch in the Sensors and Electron Devices Directorate of ARL. Dr. Wood has authored chapters in three textbooks and is

the author or coauthor of more than 100 publications in refereed journals and conference proceedings. His work includes many

patents and he is a recipient of two Army R&D Achievement Awards.

86, No. 8, 1756 (1998), and references therein.[12] D.L. DeVoe, Sensors and Actuators A 88, 263-272 (2001).[13] A.N. Cleland, M. Pophristic, and I. Ferguson, Appl. Phys. Lett.79, 2070-2 (2001).[14] P. Muralt, Integrated Ferroelectrics 17, 297-307 (1997), and references therein.[15] A. Ballato, “Mi c ro - e l e c t ro-acoustic Devices for Wi re l e s sCommunication,” IEEE Sarnoff Symposium on Advances in Wiredand Wireless Communications (March, 1999).[16] A. Gozzini, Proceedings of the XIIth Colloque Ampere, Bordeaux17-21 Sept. 1963, Ed.: R. Servant and A. Charru, Electronic Magnetic

Resonance and Solid Dielectrics.[17] J.A. Sidles, J.L. Garbini, K.J. Bruland, D. Rugar, O. Zuger, S.Hoen, and C.S. Yannoni, Appl. Phys. Lett., 58, 2854 (1991).[18] J.A. Sidles, et al., Rev. Mod. Phys. 67, 249 (1995).[19] D. Rugar, O. Zuger, S. Hoen, C.S. Yannoni, H.-M. Vieth, andR.D. Kendrick, Science 264, 1560 (1994).[20] O. Zuger, S.T. Hoen, C.S. Yannoni, and D. Rugar, J. Appl. Phys.79, 1881 (1996).[21] K.R. Thurber, L.E. Harrell, R. Fainchtein, D.D. Smith, Appl.Phys. Lett. in press (to appear, March 11, 2002).


Introduction

Miniaturization of electronics brings a multitude of benefits,particularly when a number of disparate functions are integrat-ed onto the same substrate. In the case of the ubiquitous micro-processor chip, the substrate itself is the chip. Its functions arelogic, memory, and control circuitry, which deal with theinformation flow both on and off the chip. Tremendous gainsin reliability were obtained by integrating these functions with-in the same device. These initial gains were mostly due to elim-inating connections between individual chips of first genera-tion integrated circuits, as well as reducing the total number ofparts. Since that time, miniaturization has driven up the num-ber of individual transistors on chips to well over 10 million. Ithas also resulted in a multibillion dollar industry which repre-sents a significant fraction of the US Gross Domestic Product.Integrated chips are now in most modern conveniences, such asalarm clocks, coffee makers, and most forms of public and pri-vate transportation.

The microelectronics industry is driven by two purposes: thedevelopment of new applications and the miniaturization ofexisting ones. This is why most consumers find buying a newcomputer every few years irresistible, realizing upwards of a six-fold increase in the performance for the same number of dol-lars they spent three years before. The process of miniaturiza-tion is characterized by ‘Moore’s law’, named after GordonMoore, the founder of Intel, who first noticed that the numberof transistors on a chip doubled every eighteen months.However, this is not a law of physics, but rather a law of eco-nomic survival. Companies that can’t keep up with this frenet-ic pace soon find they are uncompetitive, and then fail.Compounding this challenge is the fact that as the integrationlevel increases, the cost of manufacture per unit area of chipmust remain the same.

A version of Moore’s law is represented in Figure 1, plottingthe characteristic dimension of manufactured integrated cir-cuits (ICs) over the last forty years. Many predicted ‘ends’ toMoore’s law have come and gone. For example, at one time itwas thought impossible to manufacture a chip with feature sizesmuch below a micron. New approaches are actively beingdeveloped to allow manufacture of chip features down to 20nm (2/100 of a micron) in size.

Now we are faced with the end of Moore’s law being deter-mined by the physics of the transistors and circuits themselves,presently projected to be around 20-30 nm. Below this size, itbecomes increasingly difficult (i.e. expensive) to get these small

transistors to turn off, leading to unacceptable power dissipa-tion. While this problem may be solved by innovative devicesand circuit design, the transistor size is rapidly approaching thepoint when there are so few atoms in the critical regions of thetransistor that the underlying solid state (i.e. bulk material)physics begins to break down.

So is there a different paradigm for the manufacture andintegration of switches and memory elements? Given that weare looking for devices that have dimensions around 10 nm andlower, it is natural to turn to the chemist who after all isextremely skilled at working in this size regime. The questionthen becomes, can we design individual molecules that have aspecific electronic functionality? Anticipating success, two fur-ther questions come to mind. First, can we look for additionalhelp in assembling these molecular components into a usefulcircuit? Wiring up our molecular memory element to ourmolecular switch by direct manipulation looks hopelesslyimpractical apart from heroic onesies and twosies demonstra-tions. Second, what kind of circuits should we aim for, or morespecifically what will the architecture of our memory and logicblocks look like?

These issues also represent the three thrusts of the DARPAMolecular Electronics program and the remainder of this

Figure 1. Recent IC Developments

Have Followed Moore’s Law

Dr. Christie R. K. MarrianMicrosystems Technology Office

DARPA

100

10

1

0.1

0.01

0.001

1960 1970 1980 1990 2000 2010 2020 2030

Year

Single

First Integrated

256K

64M

16G

MOSFET (Metal Oxide Semiconductor

Field Effect Transistor) Scaling


article is framed accordingly. The last section will briefly discussthe prospects for molecular electronics and where we might seethese first commercial products emerge.

Devices

To date, chemists’ ingenuity in designing and synthesizing elec-tronically advantageous molecules has rather outpaced engi-neers’ ability to reliably test their properties at the single mole-cule level. However, this trend has begun to change as a num-ber of ways of studying electronic transport of single (or smallnumbers of) molecules have been reported. As a result, the paceof molecular design optimization is accelerating.

One of the landmark papers on conduction through singlemolecules was published by Aviram and Ratner [1]. Theypointed out that if one could couple a readily oxidizing group(D) to a readily reducing group (A), the alignment of the low-est unoccupied molecular orbital (LUMO) and highest occu-

pied molecular orbital (HOMO)would be such that electron hopping(the main mode of electronic conduc-tion in single molecules) would be eas-ier in one direction than the other( Fi g u re 2). This would result in amolecular diode. While straight for-ward in principle, this has been diffi-cult to demonstrate. Not so muchbecause of the difficulty in synthesizingthe molecules, but because of the diffi-culty in testing them.

An example from the work of JimHeath and Fraser Stoddard at UCLA[2] is somewhat more complicated and

a full mechanistic study continues. The molecule, an exampleof which is shown in Figure 3, is a member of the rotaxane fam-ily. The groups at either end of the molecule are designed tobond to the electrode surfaces. The area of interest is the centerof the molecule, which is comprised of two electron-donating(oxidizing) groups connected by a short spacer. They are sur-rounded by a ring which has a positive charge. This ring canmove up and down the backbone of the molecule and has a sufficiently positive charge to change a donor group to a netacceptor (reducing group). This motion can be induced byoxidizing/reducing the molecule. The net result is an I/V char-acteristic akin to a diode which reverses polarity! The oxida-tion/reduction is caused by raising the voltage across the endsof the molecule above a threshold (of a few volts). This is a particularly fascinating characteristic, as there is no equivalenttwo-terminal device in the solid-state world. The applicationsto memory are obvious, but Heath and his collaborators in Stan Williams’ group at Hewlett-Packard (HP) Labs are alsolooking at logic applications and are continuing to refine themolecular design.

A second example, which works on a slightly different prin-ciple, comes from the collaboration of Jim Tour (a chemist atRice Un i versity) and Ma rk Reed (an engineer at Ya l eUniversity) [3]. The I/V characteristics, presented in Figure 4,show a negative differential resistance (NDR) and is reminis-cent of a resonant tunneling diode, although it is believed to begoverned by a different mechanism. For example, it has beenproposed that the discontinuities in the characteristic are due tosuccessive oxidations of the molecule (included on the I/Vcharacteristic). This changes the conductivity of the moleculedramatically as each extra electron is taken on. The inset showsthe electron orbital calculations of Jorge Seminario at theUniversity of South Carolina [4]. He has correlated the oxida-tion states with an overlap of the electron orbitals which favorconduction in the “on” states of the molecule. In the “off” state,there is no such pronounced overlap. Interestingly, his resultssuggest a conformal change (a twisting of the central ringedgroup) associated with this conductivity change. Some experi-mental findings supporting the importance of this conforma-tional change were recently published by Paul Weiss’s group atPennsylvania State Univeristy [5]. They measured the switchingrates of Tour-Reed NDR molecules through careful Scanning

58

Figure 2. A Schematic of

a Molecular Rectifier

Figure 3. Molecular Diode

which can Reverse Polarity

Reverse

D

A

Low probability of hopping

Forward

A

D

High probability of hopping

This ring structure

may “slide” along

the molecule’s backbone

No Gradient

D

A

Metal contacts


Lumo

Homo

EF


Tunnelling Mi c roscopy (STM)measurements of individual mole-cules in a background of an inertalkane SAM (self-assembling mol-ecule). One of Weiss’ main conclu-sions was that the local order (i.e.packing) was extremely importantand that NDR molecules couldeffectively be locked into one of itsconduction states if it were notgiven “room to move” [6].

While progress in single molecu-lar devices has been impressive,they have yet to be optimized.Molecular devices developed todate have limited operating win-d ows. For example, the re s u l t sre p o rted above we re tested andevaluated at a temperature of 60K.However, this class of molecules iscurrently being re-engineered towork at higher temperatures. Another issue key to the per-formance of molecular devices is that of the end groups (oftenreferred to as alligator clips) which bind the molecules to elec-trode metals. In fact, it turns out that the current that can flowthrough an individual molecule is often dominated by conduc-tion through/across the alligator clips. To date, thiol end groupshave been mostly used as their chemistry is reasonably wellunderstood and they form stable, covalent bonds with a goldsurface. However, they by no means provide the best ‘ohmic’contacts.

Assembly

Now that chemists are enjoying a degree of success in designingmolecules with electronic functionality, the key challenge turnsto the assembly of these components into viable circuits. Oneproblem is that the molecules are so small (nanometer scale)that none of our existing tools can manipulate them at any-thing remotely close to a viable rate. Some interesting resultshave been published using proximal probes to manipulateatoms and molecules. But even the most ardent devotees of thiswork admit that it is far too slow for fabricating circuits of anycomplexity or number. Similarly, even the most advanced lith-ographic tools cannot match the resolution of moleculardevices. Instead, we must look to other ways to put molecularcircuits together. One approach would be simply to ‘synthesize’the entire circuit as a single (very large) molecule. At the pres-ent time, this is not a feasible concept, although we will prob-ably see the synthesis of increasingly large functional moleculesin the years to come.

Carbon nanotubes provide a possible framework for buildingsuch large molecules. To date, some novel devices have beenreported using the nanotubes as the conduction channel form i n i a t u r i zed transistors [7]. Charlie Lieber at Ha rva rdUniversity has also used crossed nanotubes to produce deviceswith very small active areas (<10 nm 2) [8]. However, the actualdevice sizes are dominated by the contacts to the nanotubes

being many orders of magnitude larger than the active areas.One reason for this is the difficulty in manipulating nanotubes.To fully exploit the scaling possible with this device configura-tion, the nanotubes must be positioned with nanometer preci-sion. While impressive advances in directed nanotube growthand fluidic manipulation have been made, we are still some wayfrom being able to fully exploit the benefits of these nanoscalecomponents.

We again look to the chemist for additional help in putting cir-cuits together from molecular devices. Ord e red films can beformed from disord e red solutions of monomers, designed so thatone end has an affinity for a particular substrate material. W h e nthis affinity results in a covalent bond (as in the case of amonomer with a thiol end group and a gold substrate), thep rocess is described as self-assembly. Langmuir-Blodgett films ares i m i l a r, but the monomer-substrate bond is we a k e r. In bothcases, the monomers are designed such that the densely packedfilm on the substrate is a lower energy configuration than the dis-o rd e red solution. He re, the energy is related to the bond formedb e t ween monomer and substrate. But there are other possibleenergy drivers such as surface energy, electrical energy, etc.

The challenge then becomes to design a configuration wherethe minimum energy state corresponds to something resem-bling a functional circuit. While something of the complexityof a conventional microprocessor would appear to be out ofreach, simpler building block circuits such as a memory blockor logic gate(s) could be put together in this way. Even so, itappears a series of assembly processes will be needed operatingat successively larger length scales. At this point, the research isnot as advanced as with molecular devices. Nonetheless, somefirst steps have been taken.

Under DARPA sponsorship, the Penn State Group headedby Theresa Mayer and Tom Mallouk has demonstrated at leasttwo levels of hierarchical assembly. They have been workingwith metallic nanowires fabricated by a process of templatedelectroplating. Porous membranes can be obtained with a large

Figure 4. Nitroamine

Molecule and Its Behavior

1200

1000

800

600

400

200

0

0.0 0.5 1.0 1.5 2.0 2.5

V (V)

Au

O2N

NH2

S

Au

Ipeak=1.03 nA

q = -1

Ivalley=1 pA

q = -2

T = 60K

60

number of essentially identical diameter tracks. This providesthe template for the plating. Functionality can be built into thenanowires. For example, some of the functional moleculesdescribed above can be included by interrupting the platingand self-assembling the molecules in the middle of the metallicnanowire. The plating can then be continued. The resultingfunctional nanowire can be manipulated in a number of ways.One way is to use DNA as a ‘glue’ to adhere the nanowiresselectively to a substrate (or to each other). The Penn Stateteam has also designed a novel circuit where the metallicnanowires will ‘assemble’ between two landing pads, driven bya minimization of the electrical (capacitive) energy of the sys-tem [9]. Furthermore once one nanowire has bridged the gap,the process self-limits so one, and only one, nanowire will bepositioned across the landing pads. The group is continuing torefine these assembly methods and is also looking at other tech-niques operating at larger length scales.

So a series of hierarchical assembly steps seem possible andare beginning to forge a path which could fabricate a circuitwith a degree of complexity and computational functionalityper unit area greater than extensions of Si-based microelectron-ics. However, these assembly processes are energy driven, so itseems inevitable that there will be defects such as incorrectlypositioned structures and/or molecules. Thus to be useful, thecircuit architecture must be tolerant of a significant defect level.

Architecture

Molecular memory based on molecular devices (of the typedescribed in Section 2) appears feasible. However, to fullyexploit the miniaturization benefits of molecular electronics,defect-tolerant circuits are needed. In contrast, today’s volume-produced microelectronic chips are extraordinarily intolerant ofdefects. All of the tens of millions of transistors in a micro-processor must be functional, or the chip is no more usefulthan the pile of sand from which the silicon was made. Anexample of a functional computation machine containingdefective components is the HP Teramac [10]. The key to theTeramac’s operation is the inclusion of significant redundancyof components and interconnections. Further, a period of ‘selfevaluation’ is necessary to identify and route around the defec-tive components. In other words, the machine was first puttogether and then examined to find out what was functioningusefully.

More recently, these ideas have been extended to the circuitlevel. The same group at HP that developed the Teramacrecently patented a design for a multiplexer, where the connec-tions between the address lines and the data lines are random.In a normal multiplexer, N data wires are addressed by logN

address lines, that is, each address line can be thought of as rep-resenting a power of 2 in the address of the data line. The grouppointed out that by providing extra address lines, there is a van-ishingly small probability that all the N data lines will not beaddressable even if the connections between the address anddata lines are random [11]. As the required number of addresslines scales with the logarithm of the number of data lines, fora large array, the added area taken by the extra address lines isinsignificant. A discovery phase is needed for such a circuit asthere will be address duplication (some data lines will not haveunique addresses). Thus, by providing redundancy and allow-ing for the discovery phase, it is hoped to fabricate a function-ing circuit with a non-deterministic (random) critical pattern-ing step. Many believe that fabrication of this type of randomstructure may be amenable to chemical-based assembly.

Other researchers are pursuing similar techniques to assem-ble circuits with a degree of randomness that can, nonetheless,perform a useful information processing function. The keyissue becomes the time required for the discovery phase alongwith any modifications that must be made to the structure ofthe circuit. These ideas are completely contrary to present dayfunctional circuit design. Molecular electronics offers thepotential of miniaturization and reduced manufacturing cost,with the caveat that circuit architecture will not be completelyunderstood at the outset. This is the most radical part of cur-rent molecular electronics research.

Summary: What’s Next

For a disruptive technology such as molecular electronics tomove from the research lab to commercial products, an over-whelming economic-based case needs to be made. At present,the microelectronics industry can look forward to at least adecade of continued prosperity based on miniaturization.Thus, early introduction of molecular-based technology willprobably be in a niche product that marries the advantages ofmolecular electronics with the existing Si-dominated infra-structure. An example might be non-volatile (or limited volatil-ity) memory for digital cameras and other storage-intensiveapplications. These applications require dense memories withvolatilities measured in weeks, which can be manufactured effi-ciently and economically.

The field is moving rapidly and there is considerable opti-mism as to what will be possible in the not too distant future[12]. Whether this optimism can be turned into a viable tech-nology remains to be seen. Now that there have been convinc-ing demonstrations of molecular devices with useful electronicfunctionality, the key challenges will be the assembly of thesemolecular components into functional circuits.


Acknowledgements

I would like to thank the participants in the DARPAMolecular Electronics programs who have performed the vastmajority of the research summarized here. I am also deeplyindebted to William Warren, Kwan Kwok and John Pazik withwhom I have managed these programs over the last 3 years.

References

[1] Aviram, A., and Ratner, M.A., “Molecular Rectifiers,” ChemicalPhysics Letters, 15 November 1974, pp. 277-283.[2] C.P. Collier, E.W. Wong, M. Belohradsky, F.J. Raymo, J.F.Stoddart, P.J. Kuekes, R.S. Williams and J.R. Heath, “ElectronicallyConfigurable Molecular-Based Logic Gates,” Science, (5/1999).[3] Chen, J.; Reed, M.A.; Rawlett, A.M.; Tour, J.M. “Large On-OffRatios and Negative Differential Resistance in a Molecular ElectronicDevice,” Science 1999, 286, 1550-1552.[4] Seminario, J.M.; Zacarias, A.G.; Tour, J.M. “Theoretical Studyof a Molecular Resonant Tunneling Diode,” J. Am. Chem. Soc. 2000,122, 3015-3020.[5] Conductance Switching in Single Molecules thro u g hConformational Changes, Z.J. Donhauser, B.A. Mantooth, K.F.Kelly, L.A. Bumm, J.D. Monnell, J.J. Stapleton, D.W. Price Jr., D.

L. Allara, J.M. Tour, and P. S. Weiss, Science 292, 2303 (2001).[6] The title of a favorite John Mayall song.[7] V. Derycke, R. Martel, J. Appenzeller, and Ph. Avouris, “CarbonNanotube Inter- and Intramolecular Logic Gates,” Nano Letters (26August 2001). And http://pubs.acs.org/cgi-bin/jtextd?nalefd/a s a p / h t m l / n l 0 1 5 6 0 6 f.html and Adrian Bachtold, Peter Ha d l e y,Takeshi Nakanishi, and Cees Dekker, “Logic Circuits With CarbonNanotube Transistors,” Science 294, 1317 (2001).[8] Yu Huang, Xiangfeng Duan, Qingqiao Wei, and Charles M.Lieber, “Directed Assembly of One-Dimensional NanostructuresInto Functional Networks,” Science 291, 630 (2001).[9] P.A. Smith, C.D. Nordquist, T.N. Jackson, T.S. Mayer, B.R.Martin, J. Mbindyo, T.E. Mallouk, Applied Physics Letters, 77, #9,1399 (2000).[10] James R. Heath, Philip J. Kuekes, Greg Snider, and R. StanleyWilliams, “A Defect Tolerant Computer Architecture: Opportunitiesfor Nanotechnology,” Science, 280,1717 (1998). P. Kuekes, Teramac.[11] P. Kuekes and R.S. Williams, “Demultiplexer for a MolecularWi re Crossbar Ne t w o rk (MWCN DEMUX),” US Patent #6,256,767.[12] Science magazine December 21, 2001 issue – Breakthrough ofthe Year – “Molecules Get Wired.”


Christie R.K. Marrian is currently on detail from the Naval Research Laboratory to the Microsystems Technology Office at the

Defense Advanced Research Projects Agency (DARPA) in Arlington VA. He is currently Program Manager for the Molecular

Electronics (Moletronics), Nanoimprinting (MLP) and Heterogeneous Integration (HGI) programs.

Christie obtained his B.A. and Ph. D. Degrees from the Electrical Engineering Department of Cambridge University in 1973

and 1978 respectively. He then spent three years as a postdoctoral fellow at the European High Energy Physics Center (CERN)

in Geneva, Switzerland. There he worked on the design and fabrication of a large (~2m3) detector for free quarks. In 1980, he

crossed the Atlantic to join the Surface Physics Branch at the Naval Research Laboratory (NRL) in Washington DC. Christie initi-

ated a program on Nanoelectronics in 1985. This research started with research into material modification mechanisms possible

with the scanning tunneling microscope (STM). In 1993, he was appointed Head of the Nanoelectronics Processing Facility at

the NRL where he led a group per forming nano- and micro-fabrication in a wide variety of materials for electronics, optoelec-

tronics, materials science and plasma processing. His personal research interests revolved around nanofabrication and the prop-

erties of nanometer scale structures and devices.

Christie has authored over 105 technical publications and holds 13 patents. He is a Fellow of the American Vacuum Society

(AVS) and also a Senior Member of the IEEE. He has served as Secretary of the Nanometer Science and Technology Division

of the AVS since its inception in 1992 and was elected to the AVS Board of Directors in 1997. Christie was vice chair of the 38th

AVS National Symposium in 1991 and chair of the International Program Committee of NANO 3 which was held in conjunction

with the AVS National Symposium in 1994. In 1996, he edited a special issue of the Proceedings of the IEEE on Nanometer-

scale Science and Technology. He was the Program Chair in 1998 of the 42nd International Conference on Electron, Ion and

Photon Beam Technology and Nanofabrication. In 2000, he was co-chair of NANO 6 and chair of the Gordon Research

Conference on Nanostructure Fabrication.

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Just as electronic devices continue to shrink in size and providegreater computing power and storage, photonic devices will fol-low the same trend, providing greater efficiency and function-ality. However, increasingly smaller photon emitters, detectors,waveguides, filters, and other optical components face limita-tions of scale. As the size of these devices approaches the wave-length of light (< 1 micron), it grows increasingly difficult forthem to confine photons in the dielectric structure. A promis-ing solution to this photon leakage problem is provided by theconcept of photonic band engineering, which allows the con-trol of photon dispersion, transmission, and reflection throughthe creation of periodic or quasi-periodic dielectric structures.

Photonic Crystals

Photonic crystals (also called photonic bandgap materials) aremicro-structured materials in which the dielectric constant(equal to the square of the index of refraction at optical wave-lengths) is periodically modulated on a length scale comparableto the desired wavelength of the electromagnetic radiation.Multiple interference between optical waves scattered fromeach unit cell results in a range of frequencies that do not prop-agate in the structure. At these frequencies, the light is strong-ly reflected from the surface, while at other frequencies light istransmitted.

For example, highly reflective surfaces are routinely con-structed from simple layered dielectric structures with alternat-ing layers of high and low refractive indices. One-dimensional

photonic crystals have long been used as anti-reflection coat-ings or as wavelength-selective mirrors which do not reflectheat, (such as the ones used in dentists’ work lights). In twodimensions, one can construct a periodic variation in index ofrefraction by assembling an array of cylinders, such as the arrayshown in Figure 1a. A two-dimensional photonic crystal maybe either a lattice of air columns in a dielectric material or a lat-tice of dielectric columns in air. Measuring transmissionthrough the cylinders shows a broad band where the transmis-sion drops to zero (Figure 1b).

A band gap that prohibits propagation in any direction canbe created by fabricating a structure that has the proper three-dimensional periodicity in the index, as seen in Figure 2. Thisbandgap in photon energies is analogous to electron bandgapsin semiconductors. The gemstone opal is a naturally-occurring,three-dimensional photonic crystal. The distinctive color ofopal is due to reflected light, the reflection being caused by thepartial photonic band gap of the crystal. Therefore, two- andthree-dimensional photonic crystals act as novel two- andthree-dimensional mirrors. Defects may be created inside pho-tonic crystals by removing or adding dielectric material, there-

by allowing the light propa-gation to be modified.Photonic crystals with andwithout defects have beendemonstrated at microwavefrequencies, but fabricatings i m i l a r, visible-wave l e n g t hphotonic crystals wouldre q u i re state-of-the-artnanofabrication techniques.

Waveguides

Photonic crystals are inter-esting for optoelectro n i cdevices because they makepossible the control andconfinement of photons insmall volumes (on the scaleof one wavelength cubed).The use of defects in pho-tonic stru c t u res has the


Figure 1. Two-Dimensional Photonic Crystal and Its Calculated Transmissivity [1]

Figure 2. Three-Dimensional

Photonic Crystal [2]

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.0 0.1 0.2 0.3 0.4

Normalized Frequency

2 layers

4 layers

8 layers

(a) (b)

Dr. Bob D. GuentherDepartment of Physics

Duke University

Dr. Henry O. EverittAssociate Director, Physics Division

Army Research Office

64

potential for guiding light through sharp bends in integratedoptical systems. The tighter the bends that can be made, themore compact the integrated photonic system can be. Photoniccrystals therefore provide a revolutionary capability to shrinkintegrated photonic systems dramatically.

On the other hand, photonic crystals can be drawn into opti-cal fibers for low loss, long range communication and routingof information. Two types of photonic crystal fiber have beendemonstrated. The first consists of a two-dimensional photon-ic crystal with a defect in the center (Figure 3a). The photoniccrystal is replicated in the third (non-periodic) dimensionmany meters. Light is guided down the defect and confined bythe two-dimensional photonic crystal surrounding it. The sec-ond consists of a one-dimensional photonic crystal wrappedinto a long cylindrical tube (Figure 3b). Again, light is guideddown the center and confined by the highly reflective photon-ic crystal. In both cases, light is guided down an air core, incontrast with current optical fiber which confines light in aglass core. As a result, low loss light guiding can be accom-plished at any wavelength, not just the wavelengths of lowestabsorption in glass fiber (1.3 and 1.55 microns). Reduced opti-cal nonlinearities promise much larger transmission band-widths than are currently achievable, and operation at visible

w a velengths promise much higherdetection efficiency using state of theart avalanche photodiodes.

Resonators and Lasers

The use of periodic structures to formresonators and lasers is not new. One-dimensional photonic crystals are sim-ply dielectric mirrors, and two suchreflectors facing each other form aFabry-Perot resonator. Vertical cavitys u rface-emitting lasers (VCSELs) areformed by placing a quantum we l la c t i ve region in the “d e f e c t” spacebetween such a Fabry-Perot resonator.Increased lateral confinement of thephotons is achieved through total inter-

nal reflection by etching the VCSELs into narrow posts such asshown in Figure 4a.

Using the same principle of total internal reflection, a circu-lar “whispering gallery mode” resonator can also be used as alaser cavity (Figure 4b). Such lasers have greater confinementand higher cavity energy density (Q) than conve n t i o n a lVCSELs, yielding lower lasing thresholds but weaker, omnidi-rectional emission. Slight deformations of the microdisk havepermitted more directional emission.

To gain even more control over the emission and threshold,higher dimension photonic crystal resonators may be used.Two-dimensional photonic crystals with defects have been fab-ricated in Si, GaAs and InGaAsP materials to couple filters andswitches with low loss. Point defects can be used to form lasercavities, such as a missing hole in a hexagonal array of holes, asdepicted in Figure 5. Micro-cavities with a mode volume of0.03 cubic microns have been fabricated in InGaAsP and lasingwas observed at a wavelength of 1.5 microns with opticalpumping.

These defects could potentially increase the spontaneousemission rate of photons in the visible or near-infrared region.Larger defects provide higher output powers because of thegreater active material volume. The large cavity shown in

Figure 3a Figure 3b

Figure 3. Photonic Crystal Optical Fiber [3] and Integrated Photonic Waveguide [4]

Figure 4. Vertical Cavity Surface Emitting Laser [5] and Microdisk Laser [6]


Figure 4a Figure 4b


Figure 5 has modes similar to the whispering gallery modes ofthe microdisk laser (Figure 4b).

Quantum Information Science

One of the most exciting potential applications for nanopho-tonic devices may be the demonstration of the first quantuminformation processing units. Unlike classical bits of informa-tion which must be either in the “0” or “1” state, quantuminformation is based on the laws of quantum mechanics whichpermit a “quantum bit” or “Qbit” to be in a superposition ofboth states simultaneously. Recently, an all-optical scheme forperforming quantum computing was proposed using linearoptical components and the nonlinear measurement process. Aversion of this quantum computer may someday be construct-ed using photonic crystal-based nanophotonic components.

Another goal of quantum information processing is to trans-

port quantum bits and teleport quan-tum information over optical fiber. Todo this, two new nanophotonic devicesare needed: single photon sources andlow loss optical fiber. Recently, groupsat St a n f o rd Un i versity and theUniversity of California, Santa Barbarademonstrated the first semiconductorsingle photon sources based onVCSELs and microdisk re s o n a t o r s ,respectively. Likewise, sources of entan-gled photon pairs and atomic or solidstate systems for quantum logic andquantum memory may work more effi-ciently at wavelengths other than 1.3 or1.55 microns. Low loss photonic crystal

fiber may prove to be ideal means for transporting and preserv-ing these fragile photonic quantum states in the visible wave-length region.

References

[1] Pictures taken with the kind permission from the web siteof A.L. Reynolds (www.areynolds.com) Copyright © 2000Photonic Band Gap Materials Research Group, University ofGlasgow.[2] Courtesy of Axel Scherer, Cal Tech.[3] Courtesy of Crystal Fibre and A. Gaeta, Cornell University.[4] Courtesy of John Joannopoulos, MIT.[5] Courtesy of Axel Scherer, Cal Tech.[6] Courtesy of Atac Imamoglu, University of California, SantaBarbara.[7] Courtesy of Axel Scherer, Cal Tech.

Figure 5a

Figure 5b

Figure 5. Single Defect and Large Defect Cavities In 2D Photonic Crystals [7]

Dr. Henry Everitt ser ves as associate director for the Physics Division of the Army Research Office. He is a program manager for

the Army’s extramural 6.1 basic research program in experimental condensed matter physics. His program specializes in research

involving quantum information science, nanophysics, and nanooptics. He serves as chairman of the US quantum information

science coordinating group and has recently ser ved as spokesman for the Army’s nanoscience program.

Dr. Everitt is also an adjunct professor of physics at Duke University. There he directs a research laboratory in which the ultra-

fast optical properties of wide bandgap semiconductors are investigated. His students also study photonic cr ystals, molecular

collision dynamics, far infrared lasers, ultrafast optical techniques, astronomical interferometric imaging, and variable stars in

globular clusters.

Dr. Bob D. Guenther received his undergraduate degree from Baylor University and his graduate degrees in Physics from the

University of Missouri. He has had research experience in Condensed Matter and Optical Physics. For nine years he was active

in Research Management as a Senior Executive in the Army, responsible for the Physics Research sponsored by the Army. In that

role he managed a program of about $20M/year involving about 100 individual projects. He also served as a senior advisor

to the Army on a variety of issues such as electric gun technology, high energy lasers, and automatic target recognition. From June

1997 through May 1999, he held the position of Interim Director of the Duke Free Electron Laser Laboratory.

66 The AMPTIAC Newsletter, Volume 6, Number 1

23rd International Conference on Microelectronics5/12/02 – 5/15/02Nis, YugoslaviaMr. Vojkan Davidovic, MIEL 2002TreasurerFaculty of Electronic EngineeringUniversity of NisBeogradska 1418000 Nis, Yugoslavia

Magnetic Nanostructures5/12/02 - 5/17/02Barga, ItalyR. CelottaNational Institute of Standards

&Technology100 Bureau Drive, MS 8412Gaithersburg, MD 20899-8412Email: [email protected] Link: www.grc.ur.edu

First International Conference on Nano-scale/Molecular Mechanics (N/MM-I)05/12/02 - 05/18/02Maui, HIM. RoukesCaltechMC 114-36Pasadena, CA 91125Fax: (626) 683-9060Email: [email protected]

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7th International NanotechnologyConference6/24/02 – 6/28/02Malmo, SwedenLennart HakanssonMalmo Congress BureauOstergatan 3S- 211 25 MALMOSwedenEmail: [email protected] Link: www.malmo-congress.com

SPIE 47th Annual Meeting, "Materials and Nanotechnologies"07/07/02 - 07/11/02Seattle, WASPIEP.O. Box 10Bellingham, WA 98227-0010Fax: (360) 647-1445Email: [email protected] Link: www.spie.org

Faraday Discussion 125: Nanoparticle Assemblies7/14/03 – 7/16/03University of Liverpool, UKChristine HallConference and AwardsRoyal Society of ChemistryBurlington HousePiccadilly, London W1J OBAPhone: +44 (0) 20 7437 8656Fax: +44 (0) 20 7734 1227 Email: [email protected] Link:www.rsc.org/lap/confs/fara125.htm

SmallTalk 2002: The Microfluidics,Microarrays & bioMEMs Conference& Exhibition7/28/02 – 7/31/02San Diego, CAsmallTalk 20021201 Don Diego AvenueSante Fe, NM 97505Phone: (505) 988-5326Fax: (505) 989-1073Email: [email protected] Link:labautomation.org/smallTalk/ST02

Workshop on Nanostructures forElectronics & Optics – NEOP8/18/02 – 8/21/02Dresden, GermanyInst. of Ion Beam Physics

& Materials ResearchP.O. Box 51 01 1901314 Dresden, GermanyFax: +49 351 260 3285Email: [email protected] Link: www.neop.de

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IMAPS 20029/4/02 – 9/6/02Denver, COInternational Microelectronics

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